Firefox Sync: How to Set Up and Sync Open Tabs Between Devices

Firefox Sync is Mozilla’s answer to the “I left my tabs on the other computer” problem: sign into the same Mozilla account on two or more devices, and your bookmarks, saved passwords, and open tabs show up everywhere. Turning it on takes about a minute — no third-party app, no manual export, no emailing yourself a link. This guide walks through exactly how to set it up, what you can choose to sync (and what you might want to leave out), how to add a phone, a second computer, or a brand-new replacement machine, and what happens when a device gets signed out. It also covers something Mozilla is upfront about: Sync keeps your data current, but it was never built to be a backup. Near the end, we’ll look at where that gap shows up in real life, and how a free tool like BelayTab closes it for anyone who treats an open window as a working document, not a throwaway.

What Is Firefox Sync and What Does It Actually Sync?

Firefox Sync is a free, built-in feature that keeps your bookmarks, passwords, open tabs, browsing history, payment methods, add-ons, and settings identical across every device signed into the same Mozilla account. You don’t install anything extra for it. It’s already part of Firefox, on desktop and mobile. The trigger for all of it is one account; sign into that account anywhere Firefox runs, and Sync turns itself on automatically, no separate switch to flip.

A Mozilla account is a bit like a Google account is to Chrome: a login Mozilla uses to identify you, free to create, and separate from whatever email provider you already use. Some people search for this feature as “Mozilla Sync” rather than “Firefox Sync”. It’s the same thing, the same free account behind it. If you’ve never explicitly signed in anywhere, nothing is currently syncing: each device is running on its own local history, tabs, and bookmarks until you connect them.

Everything below assumes Firefox is already installed on each device you want connected. If you’re still weighing whether Firefox is the right browser for you, our Firefox overview covers the basics before you commit an account to it. And if you’re arriving here because a window just vanished and you’re not sure Sync will help, keep that question in mind. We’ll answer it directly a few sections down, because the honest answer is more nuanced than “yes, Sync has you covered.”

How Do I Turn On Firefox Sync?

You turn on Firefox Sync by clicking Sign in in the Firefox menu and logging into, or creating, a Mozilla account — Sync switches itself on the moment you’re signed in. There’s no separate installer and no subscription; one signed-in account does the whole job.

Once that’s done, Firefox handles the rest quietly in the background. You won’t see a progress bar or a “syncing now” banner most of the time. It just keeps working, the same way autosave does in a document. If you already have a Mozilla account from another Firefox install, signing in with that same email is faster than it sounds: Firefox recognizes it immediately and skips the confirmation-code step entirely, since that only applies to brand-new accounts. There’s no credit card, no plan to pick, and no trial that expires — the free tier is the only tier, and it covers everything Sync does.

How Do I Make Sure Open Tabs Are Syncing?

Open tabs sync automatically once Sync is on, but you can confirm (or turn it off) from Settings, in the Sync section, where Open tabs sits alongside Bookmarks, Passwords, History, Payment methods, Add-ons, and Settings as its own toggle.

The simplest way to check it’s working is the direct one: open Firefox on your other device and see whether the tabs you left open on the first one showed up. Give it a moment first — Sync runs on a background interval, not instantly on every single click, so a tab you just opened might take a beat to appear elsewhere.

The most common miss. If tabs aren’t showing up anywhere, double-check that Open tabs is toggled on in Settings > Sync on both devices, not just one. It’s easy to leave it off on a second computer without noticing.

This is the setting most people actually came here for. Bookmarks and passwords tend to get set up once and forgotten, but open tabs are the thing you check daily — the half-read article on your laptop that you want waiting on your phone at lunch, or the forty research tabs from a desk computer you want mirrored on a home machine that evening. Because it’s just one toggle among several, it’s worth treating it as its own checklist item rather than assuming “I turned on Sync” automatically covers it on every device you own.

How Do I Choose What Syncs in Firefox?

You choose what syncs from Settings, in the Sync section, where each item can be switched on or off on its own. You don’t have to sync everything just to get open tabs shared between devices.

There’s a real reason to be selective here, not just tidiness. A shared or work computer is a fair place to leave Passwords and Payment methods switched off while still syncing Open tabs and Bookmarks. You get the convenience without putting saved card numbers on a machine other people touch. History is another one worth a second thought: syncing it means a search from your personal laptop shows up in the history of a work machine signed into the same account, which is fine for some people and not for others. None of these choices are permanent; every toggle can be flipped back the next time you’re in Settings, and the change takes effect on that device without disturbing what’s already synced elsewhere. If you’re weighing Firefox against Chrome for this exact kind of control, our Chrome Sync guide breaks down what that browser syncs and doesn’t.

How Do I Connect a Second Computer to Firefox Sync?

You connect a second computer by opening the Firefox menu, choosing Sync and save data, and signing in with the same Mozilla account you used on the first device — no code to type in, no pairing step.

1. Install Firefox on the second computer if it isn’t already there.
2. Open the menu, then click Sync and save data.
3. Sign in with the same Mozilla account — same email and password as your first device.
4. Check Settings > Sync on this device too, so the items you care about, especially Open tabs, are toggled on here as well.

Worth flagging: a Firefox profile and a Mozilla account are two different things. A profile is local storage on one computer, holding that computer’s bookmarks, passwords, and preferences before Sync ever gets involved. Your Mozilla account is the layer that connects separate profiles on separate computers. If you run multiple profiles on one machine, our guide to Firefox profiles explains where each one lives and how they interact with Sync. A common point of confusion: adding a second computer doesn’t merge the two profiles into one, it just gives them a shared account to exchange data through. Each computer still has its own local copy, kept in step by Sync rather than replaced by it, which is why a setting toggled off on one machine has no effect on the other until you change it there too.

How Do I Set Up Firefox Sync on Android or iPhone?

You set up Sync on mobile by installing Firefox for Android or Firefox for iOS and signing in with the same Mozilla account you use on desktop — the apps use the same sign-in flow, just linked to your account separately from the desktop step.

Once you’re signed in on the phone, it’s worth opening Settings there too and confirming Open tabs and Bookmarks are toggled the way you want — mobile defaults can differ slightly from desktop, and it’s easy to assume they matched automatically when they didn’t. People frequently set up Sync on a laptop, forget about it for months, then get a new phone and wonder why nothing carried over; the answer is almost always that the phone was never signed in at all, since desktop and mobile setup are genuinely two separate steps that share the same account rather than one setup that covers both. One thing worth flagging early: BelayTab, the tool we keep coming back to in this guide, is a desktop browser extension. It doesn’t run on Android or iPhone. For phone tabs specifically, Firefox Sync is genuinely the right tool for the job, full stop.

Is Firefox Sync a Backup for My Tabs and Bookmarks?

No — Mozilla is explicit that Sync “does not provide a true backup service, nor is it intended to be used as one,” because it continuously mirrors whatever changes you make rather than keeping independent, restorable snapshots.

Bookmarks do get one extra safety net Firefox builds in separately from Sync: it automatically keeps up to 15 dated JSON backups in a bookmarkbackups folder inside your profile, and you can restore one from the Library window’s Import and Backup menu. That restore is worth doing carefully, though — picking a backup and restoring it overwrites whatever bookmarks currently exist on that device, so it’s an emergency move, not something to click through casually just to see what’s in an old backup. Open the Library faster with Ctrl + Shift + O (Cmd + Shift + O on Mac) instead of digging through the menu. For a step-by-step on that process, see our guide to exporting Firefox bookmarks, and if you’d rather see exactly where that profile data lives on disk without hunting for it, our browser file locations tool lists it by operating system.

Open tabs don’t get that same dated-backup treatment. Once a window is gone (closed by accident, lost to a crash, wiped during a device reset) Sync has nothing to roll back to, because it was never keeping a separate copy in the first place. That’s the gap this whole guide keeps circling back to, and it’s also why BelayTab exists: automatic snapshots of every open window on a schedule, kept as real version history you can restore from, not just a live mirror. If a window closed before you ever got to Sync, our guide on restoring your last Firefox session covers what the browser itself can still do for you.

How Do I Set Up Firefox Sync on a Brand-New Computer?

You set up Sync on a new computer the same way you’d set it up anywhere: install Firefox, sign in with your existing Mozilla account, and let Sync pull your bookmarks, passwords, and open tabs onto the fresh install — but a new machine adds a couple of extra decisions worth making deliberately instead of on autopilot.

1. Install Firefox on the new computer from Mozilla’s site.
2. Open the menu and sign in, entering the same email and password you use on your existing device. This is an existing account, so there’s no confirmation code to wait on.
3. Open Settings > Sync and check every toggle you want active on this machine. A fresh install doesn’t inherit your old device’s choices automatically; it starts from Firefox’s defaults until you set it.
4. Give it a minute, then look. Open a new tab or check your bookmarks toolbar and confirm your existing data has arrived before you start rebuilding anything by hand.
5. Decide about the old computer. If it’s being retired, sign out of it deliberately and choose whether to keep or delete its local data, rather than leaving it signed in and unused.

This is also the moment people most often lose something without realizing it, because it’s tempting to treat a new computer as a clean slate and start bookmarking and organizing tabs from scratch before checking whether Sync already brought everything over. Do the sync check first. If dozens of tabs from the old machine were part of an ongoing project rather than just clutter, it’s worth saving them as an organized, named group before you retire that computer — Sync will carry the open tabs over, but it won’t label or group them for you the way a dedicated save does. Our guide to saving tabs covers that kind of one-click, named save if you’d rather not rely on “whatever happened to be open” surviving the move.

How Do I Disconnect a Device or Sign Out of Sync?

You disconnect a device by opening the Firefox menu, clicking your account email at the top, and choosing Sign out — Firefox then asks whether to keep or delete that device’s synced data.

This matters more than it sounds like. Selling a laptop, handing a phone down to a family member, or walking away from a shared work computer are all good reasons to sign out deliberately rather than just closing the browser. Choosing “delete” wipes that device’s copy of your synced bookmarks, passwords, and history, which is the right call before it changes hands. Choosing “keep” leaves the local copy intact but stops it from receiving further updates — useful if you’re just stepping back from syncing that one machine without erasing what’s already on it. It’s also worth doing this before a factory reset or an operating system reinstall rather than after; once a device has been wiped, you’ve lost the chance to choose, and Firefox on the account side will simply see that device go quiet.

If you’re troubleshooting a device that used to sync and doesn’t anymore, it’s worth checking whether someone (possibly a past you) signed it out on purpose. Signing back in with the same Mozilla account picks Sync back up from where it left off, pulling in whatever has changed on your other devices since you were last connected.

Firefox Sync vs Chrome Sync: What’s the Difference?

Both sync bookmarks, passwords, open tabs, and history across devices tied to one account — the practical difference is mostly which account system you’re committing to, not what’s fundamentally possible with either one.

We cover Chrome’s side of this in detail in our Chrome Sync guide and our full Chrome vs Firefox comparison, which are worth reading if you’re deciding between the two browsers. The short version for this post: Firefox Sync and Chrome Sync don’t talk to each other, so if you use both browsers day to day, signing into one doesn’t help the other. Picking a primary browser (and one account system to live in) is the real decision here, more than any specific setting either one offers.

If you deliberately split time between browsers, the calculus changes. Plenty of people keep Firefox for personal browsing and Chrome for a work profile, or switch between the two depending on which extensions a task needs, and in that case neither browser’s native Sync helps at all, because each one only syncs within itself. That’s a genuinely different problem than choosing a primary browser, and it’s the specific gap that a cross-browser tool has to solve rather than either vendor’s own sync feature.

What Do I Do When Firefox Sync Stops Working?

Start by checking Settings > Sync on both devices to confirm the items you expect, especially Open tabs, are still toggled on — a setting switched off after an update is the single most common cause of “my tabs stopped showing up.”

From there, work through it in order:

• Confirm you’re signed into the same Mozilla account on both devices, not two different accounts by mistake — easy to do if you’ve had Firefox installed for years and forgot which email you used originally.
• Check your internet connection — Sync needs it to push changes in either direction, and a device that’s been offline for a while will simply be behind until it reconnects.
• Sign out and back in on the problem device to force a fresh sync rather than waiting for the background process to catch up on its own.
• If a device was disconnected on purpose earlier, you’ll need to sign in again and re-toggle what you want synced, since those toggles reset per device rather than carrying over from your account.

Beyond Firefox’s own settings, BelayTab also has its own optional paid Sync feature ($4 a month or $36 a year, with a 30-day refund window) that works across different browsers you sign into, not just Firefox, and keeps 90 days of cloud version history plus web access at app.belaytab.com. It’s a separate thing from Firefox Sync, not a replacement for it — Firefox Sync is free and handles Firefox-to-Firefox; BelayTab Sync is for when your tabs need to follow you across browsers, or when you want restorable history that outlives a single mirrored state.