Until an expert shows-up .... look at Bookmarks -- DOES Manage Bookmarks show? If YES /clk Import & Backup/ clk Restore .... do you see a column of bookmark bkups to Select one by date?

I use an app - Hekasoft Backup & Restore -- that amounts to a 5-10 second backup of the FF Profile, including Bookmarks. You NEED some Profile Backup schedule for such issues as this. Get some help if needed to install and use it - OR - just periodically Copy-Paste the FF Profile to a safe place.... https://hekasoft.com/hekasoft-backup-restore/

In the above Import-Export section Export allows bookmarks.html to be Backed-up to any folder you make. Importing these back for problem fix Adds TO what is already there / Restore erases what's there and writes in what is in the backup date chosen.

Can you open the Bookmarks Manager (Library) via the "Ctrl+Shift+O" shortcut  ?

• https://support.mozilla.org/en-US/kb/bookmarks-firefox

See also:

• https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks
• https://support.mozilla.org/en-US/kb/recover-user-data-missing-after-firefox-update