http://kb.mozillazine.org/Unable_to_process_the_backup_file_-_Firefox

jscher2000 has a tool on his website to extract out your bookmarks and save them to an HTML-format bookmark export file. If you want to try it, here is the page:

https://www.jeffersonscher.com/res/bookbackreader.html

To open the bookmarkbackups folder in Finder, use the "Open in Finder" button in the first table on the Troubleshooting Information page, and double-click into bookmarkbackups where you should see several dated files. From there you can drag and drop onto the conversion tool.

To import the HTML file, see the steps in this article: Import Bookmarks from an HTML file.

Back up and restore information in Firefox profiles.{web link} I use an add-on that makes backups of my user profile, Just In Case. Unfortunately, it is not compatible with the current Firefox.

Every now and then, open the profile folder and create a copy of the current profile as is to another location on your hard drive. This way you will have a backup.

If you do this each day, you will not lose much data. This should be done with Firefox Closed.

Note: In case you need to restore from these backups, you may need to overwrite old or corrupted files.

Thanks for your help - good advice!

Regarding an immediate solution, after posting my question did a bit more digging and found some advice that just suggested quitting FF and restarting. I did this and all my bookmarks were returned :)

Now to back them up...

That was very good work. Well done.