Whoa, that's hard to explain. If you check the available bookmark backups, are there any recent dates listed? This article will get you to that menu: Restore bookmarks from backup or move them to another computer.

Also, did any of your computers have an "Old Firefox Data" folder appear on the desktop? That would be created during a Firefox Refresh, if that happened.

Hi, I'm not much of a computer geek, just like to turn them on and use them! These computers are not networked or synced so it must be upgrade-related, has anyone else reported this? The Win 7 PC has a new file "old Firefox Data" on the desktop, looks the same as the current file. I can't see any backups. Does that make sense?

Routine updates should never erase data. If the installer detects that there has been a long delay since the last update, they may suggest a Tune-up (Refresh). In the Old Firefox Data folder, if you double-click into the folder inside and then into the bookmarkbackups folder, do you find anything recent there (the creation dates are coded into the file names)?

Did your Windows 10 computers get a Windows update, or only the Firefox update?

We fixed the problem on the Win 10 PCs by doing system restores. No idea what caused it, probably updates, but strange the problem was on several PCs. Thanks for your help.