This morning, I "upgraded" Firefox 60.0.2 to 61.0.0 on my desktop computer (Linux Mint). When I started the new version, it came to life with the correct default tabs and pages, no menu bar (my reference), and with the bookmark tool bar visible ... but completely empty. Toggling the menu option to display it was unproductive. I restored the most recent backup of the bookmarks, but the tool bar remained empty.
So I killed Firefox, started it in safe mode (no change), then killed it again and restarted it normally. This time the bookmark tool bar was populated with the correct bookmarks and folders. (I don't know if passing through safe mode was necessary. Maybe it just needed another restart after the restoration operation.) Unfortunately, my problems were not yet over. Although I had the correct top-level stuff in the bookmark tool bar, the various folders only had about three items each, regardless of how many were supposed to be in each folder (and, trust me, it was typically more than three).
When you go to restore bookmarks in Firefox, it will show you a list of backup files (I think it keeps the fifteen most recent) and how many items each contains. My recent backups were all listed with 18 to 21 items. Fortunately, I also have Firefox (not yet upgraded) on my laptop (running the same version of Linux Mint), with the same bookmarks. On the laptop, recent backups have 444 items. So either the upgrade messed up the backup files or Firefox 61.0.0 has trouble reading backups from version 60. Heck, maybe Firefox 60 screwed up making the automatic backups on my desktop (but, somehow, not on my laptop).
The laptop proved my savior. I manually backed up the bookmarks on it to a file, parked that file on Dropbox just in case, copied it to the desktop and manually restored it. For the moment, at least, I have all my bookmarks back.
In case you're reading this because you're in the same boat, here are the steps to do manual backups. Of course, this will only help if you have your bookmarks intact somewhere. If you're thinking of upgrading Firefox but haven't pulled the trigger yet, you might want to make a manual backup for insurance.
Start with the "hamburger menu" (the button three horizontal parallel lines). From there, click the "Library" option, then "Bookmarks", then "Show All Bookmarks" (at the very bottom). That opens up a window titled "Library". Click the "Import and Backup" drop-down menu, then either "Backup" or "Restore" depending on your intent. Backup will give you a typical file saving dialog. Restore will give you a list of your recent backups and an option at the bottom to select a file. Use that option to navigate to a manual backup.
Once again, software saves me from having a productive morning. :-(
By the way, this bug has already been reported: https://bugzilla.mozilla.org/show_bug.cgi?id=1472127.