Sorry to hear you're having trouble, but this is a volunteer forum. People will respond when they have the time - and if the question is something they feel they can or want to help with.

The only automated function I can think of that would mess with bookmarks and settings is Sync. Are you using Sync in Firefox? Other than that, I can only imagine this happening if an older profile is selected or the system state and your profile files along with it get reverted to an earlier state (automated backups, e.g. Time Machine).

Bookmarks do get automatically backed up, but you'd have to manually roll them back to land with old bookmarks. You can try restoring newer versions, too: https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them?redirectlocale=en-US&redirectslug=Backing+up+and+restoring+bookmarks#w_restoring-from-backups.

As far as I know, settings have no rollback functionality, they are realtime.

Are you using Sync?

cor-el said

Are you using Sync?

I do seem to have sync 'on'. Would you recommend that I turn it off? I can see how this might cause some problems, but I don't actually use Firefox on any other device. Having checked the backups, I could see that I had fewer bookmarks 2 weeks ago so I was reluctant to revert to a previous version in case I lost my most recently bookmarked pages. The only solution was to re-organise the bookmarks manually again. I just hope I won't continue to have problems.

Overall, if you aren't using Sync on any other devices disconnect it and don't use it at all. Sync isn't meant to be a "cloud" for backup purposes.

It is best to disconnect from Sync before rearranging bookmarks in a big way. Shouldn't be an issue if you're only moving a few bookmarks around, but for larger moves it is best to shut off Sync until you have completed your editing.