Sorry, there is no '''undo''' with Sync. Once Firefox data is merged on a device, there's no way to remove that data without removing the "original" data that belongs there. ''But there are backups for bookmarks going back like 15 days. See below a bit. %'' Once a 2nd device is connected to Sync, that device will get sync'd with the Firefox data that is on the Sync server and will sync the data from that 2nd device to the Sync server. Then the next time the 1st device has Firefox opened, the data from Sync & the 2nd device will be merged on the 1st device. % To "fix" what happened as far as Bookmarks goes, '''Disconnect...''' the affected device from Sync. (In Options > Sync = Disconnect) Then open the Library window {Ctrl+Shft+B} or by using '''Show your bookmarks''' from the "clipboard" button to the right of the Star button, then select '''Show All Bookmarks'''. Using the '''Import and Backup''' menu item at the top, hover over '''Restore''' and then select the day previous to the day that "merge" was done. Select that Backup date to "restore" - only the bookmarks from that day, on that device will be "restored". That bookmark data will replace the bookmark data that is currently within that Firefox installation for that user. When you go to '''Reconnect''' to that Sync account - '''don't''' reconnect until you change the Password for that Sync account here. https://accounts.firefox.com/settings ''Near the bottom just above Delete account - where it says '''Password --Change...''''' Changing the account Password will clear all data for that account on the Sync server. Otherwise if you reconnect to Sync with the existing Password, all those bookmarks will return to that device. '''Or''' maybe it would be better to just Delete that Sync account and start out all over again with a new Sync account ... The main thing is to not re-connect to Sync using the same account Password. And that is true for all devices that your husband may have connected to that Sync account. In conclusion, that only works for Bookmarks. Passwords, Browsing History, Tabs, Add-ons, and Preferences have no automatic back-ups saved and those data types will need to be deleted one-by-one if those types of data were included in the Sync process.