Are you able to add bookmarks, or is bookmark storage "frozen"?
First, if you haven't already, please do a normal shutdown and/or restart of your system. Restarting can help by freeing up locked files, completing any deferred updates, and resolving temporary file permission issues.
If Firefox still has bookmark problems, try the "Verify Integrity" diagnosis and repair function. That's on the "Troubleshooting Information" page (https://support.mozilla.org/kb/use-troubleshooting-information-page-fix-firefox😞
Near the bottom, there is a section titled Places Database -- that's the file which stores history and bookmarks. Click the button labeled Verify Integrity. There may be a 10-15 second delay before results appear.
If all is well, the report that appears below the button should start with:
> Task: checkIntegrity
+ The places.sqlite database is sane
+ The favicons.sqlite database is sane
> Task: invalidateCaches
+ The caches have been invalidated
> Task: checkCoherence
+ The database is coherent
...
Sometimes that will fix things automatically, sometimes not.
If yours reports any kind of problem reading files, or corrupted files, I suggest heading over to the Mozilla Support site. They have a team of support volunteers to troubleshoot malfunctions with your currently installed version of Firefox, while this site is focused on idea submissions for future versions of Firefox (looking ahead anywhere from 8 weeks to many months). Here's a link to the new question form (for Windows/Mac/Linux): https://mzl.la/3y1vQUQ
