Stop Firefox Sync From Erasing Local Data

Signing in to Firefox Sync on a second computer can replace the bookmarks, history, passwords, and open tabs already stored there. Nothing is deleted from your account, but the local version of each enabled data type can be swapped for the account copy.

Back up bookmarks and logins first, disconnect the account, and decide which categories are allowed to sync before you reconnect the device.

In this guide, I have covered:

Why Sync Can Replace the Data Already on Your Computer

You sign in to your Firefox account on a second computer, and within a minute the bookmarks or history on that machine look wrong. Nothing crashed. Sync worked exactly as designed, in a direction you did not expect.

Firefox Sync keeps the selected data types identical across devices. When you connect an account to a profile that already holds data, Firefox compares both sides and can replace the local copy of an enabled type with the account copy.

The profile itself stays on disk. What changes is the local version of your data: bookmarks, history, open tabs, saved passwords, addresses, payment methods, add-ons, or settings. So the time to act is before you connect the computer, not after.

Mozilla’s own account and Sync guidance is worth reading for the official wording: Firefox support documentation.

Back Up Local Data Before You Touch Anything

Bookmarks, logins, and settings only count as safe once a copy of them exists outside the profile. Make that copy before you sign in, disconnect, or change any account setting.

1. Click the menu in the Firefox toolbar.
2. Select Bookmarks, then Manage bookmarks to open the Library.
3. Use the Library’s export option to save the bookmarks as an HTML file.
4. Open Passwords from Firefox’s saved logins page and confirm the important passwords are also stored somewhere else, or export them before you continue.
5. Note down any local history, open tabs, addresses, add-ons, or settings you do not want replaced.

If the saved logins screen is unfamiliar, Add, Edit, and Manage Passwords in Firefox on the Computer walks through where everything sits.

A bookmark export protects the bookmarks file, but it is not a complete backup of every profile item. Pay particular attention to passwords, payment methods, and locally installed add-ons, because those are the hardest things to recreate.

Disconnect the Account Before You Connect Another Device

If this computer is already signed in, cut that link first. Disconnecting stops the profile from syncing while you inspect and prepare it, and it leaves the account and the data stored for it in Firefox services untouched.

1. Open Settings, then select Account and sync.
2. Select Manage account.
3. Select Disconnect to remove the account from this computer.
4. Confirm the disconnection if Firefox shows a confirmation prompt.
5. Check the account panel again and verify it now shows that you are not signed in.

You’ll know it worked when Account and sync shows You’re not signed in instead of Manage account or Syncing is ON.

If Disconnect is not visible, type “disconnect” into the Find in Settings box, or open about:preferences#sync straight from the address bar.

Choose Which Data Types Can Sync

Not every category deserves a trip through the cloud. If local passwords or payment methods matter more to you than having them on every device, switch those categories off before you reconnect.

1. Open Settings, then Account and sync.
2. Select Manage synced data.
3. Review the listed categories: Bookmarks, History, Open tabs, Passwords, Addresses, Payment methods, Add-ons, and Settings.
4. Clear the checkbox beside any category that holds local data you do not want synchronized.
5. Select Save.

Passwords, payment methods, and settings deserve the most thought. Leave them enabled and account data stays in play for those categories.

“Manage what syncs on all your connected devices” changes the account-wide choices. “Manage what syncs on this device” applies to the current computer. Check the title of the dialog before saving so you do not change the wrong scope.

Sign In Without Turning Sync On

Sometimes you want the account itself, for a Mozilla service or a support thread, without handing your browsing data over. Firefox separates signing in from syncing, so you can do one and skip the other.

1. Open Settings, then Account and sync.
2. Select Sign in to sync… and complete the account authentication.
3. Look for the choice to turn syncing on, shown as Turn on syncing… or Turn on syncing.
4. Leave that choice alone until you have checked the categories that will be included.
5. Verify the panel shows you are signed in but that Syncing is OFF.

If Firefox starts syncing the moment you sign in, disconnect again, open Manage synced data, switch off the affected categories, then reconnect.

For a one-off session on a shared computer, a private window is the safer habit. Private sessions leave no history or open tabs for Sync to compare in the first place.

Keep the Account Inside a Separate Profile

When you genuinely need this account on this machine, a second profile gives the account somewhere to live that is not your everyday browsing. A copied profile does not carry the account or Sync information from the original.

1. Open Settings, then General.
2. Select Manage Profiles.
3. Select the option to create a new profile.
4. Give the new profile a recognizable name and finish creating it.
5. Open Account and sync in the new profile and sign in to the existing Firefox account.
6. Let the account data sync inside the new profile only.

You can also open about:preferences#profiles directly. Once the account is connected, return to your original profile and confirm its local bookmarks, history, and saved information are still there.

Profiles live inside Firefox’s stored data, so deleting those files wipes every profile at once. If you ever uninstall Firefox and delete its stored files, export the profile you want to keep first.

TIP: Before connecting the account, place a temporary bookmark with a distinctive name in the original profile. After signing in, check whether that bookmark remains. If it disappears, disconnect Sync before making further changes.

Reconnect the Device in the Right Order

Backup made, categories chosen, profile protected. The connection itself is now the low-risk step.

1. Open Settings, then Account and sync.
2. Select Sign in to sync… and sign in with the Mozilla account you intend to use.
3. Verify the account email before continuing.
4. Select Turn on syncing… only if you intend to synchronize the categories you picked.
5. Select Sync now if you want Firefox to begin the selected synchronization immediately.
6. Confirm the local data you want to keep is still present.

If something was replaced anyway, select Disconnect immediately, then restore the backed-up bookmarks or other items from the copy you made before signing in.

What Not to Do

Most Sync accidents come from a handful of habits. Skip these and you skip the recovery work entirely.

• Signing in just to test whether an account works is how profiles get overwritten. Test in a separate profile instead.
• Syncing is OFF does not mean the computer holds no account information. Disconnect the account when it must stay inactive.
• Remove Account is not the same action as Disconnect. Use it only when you actually want the account gone, never to stop syncing on one computer.
• Deleting the server account because the local copy changed solves nothing, and it can cost you the version that still exists in the account.

Check Which Devices Are Connected

An unfamiliar device on the list explains data you never added. The list sits in the account management area, not in any local profile.

1. Open Settings, then Account and sync.
2. Select Manage account.
3. Look for Connect another device and the device names listed near it.
4. Disconnect the current computer if it should not remain connected.
5. Check Device name and use Change Device Name if the name tells you nothing.

This check shows which computer or device is attached to the account. It does not undo a local replacement that already happened.

Recover if the Data Was Already Replaced

Bookmarks or history already changed? Stop the sync before you do anything else. Every extra sync makes it harder to tell which version you are looking at.

1. Open Account and sync and select Disconnect.
2. Restore the bookmark HTML export through the Library’s import or restore option.
3. Compare the local bookmarks with the backup and confirm the missing items have returned.
4. Reconnect the account only after limiting the synchronized categories and testing the setup in a nonessential profile.

For saved passwords, open the Firefox saved logins screen and check whether the expected entries are still there. If they are gone, restore them from your separate password backup or password manager.

RELATED: Open Private Window and New Tabs in Firefox on the Computer