I installed the latest update today. Jan 12. I lost all my bookmarks. This also happened the last time there was an update a few months ago. Maybe 100 bookmarks collected over 15-20 years gone.
I didn't even want an update.
Look on your desktop. Do you see a folder called; Old Firefox? Look inside. Look for the folder with the latest creation date.
Type about:profiles<enter> in the address bar. How many profiles are listed? How many should be there? Also, open the profile folder in your file explorer.