see ...

• I can not get Google Toolbar On my new firefox 5 !!!!!!! | Firefox Support Forum | Firefox Help
  https://support.mozilla.com/en-US/questions/837473
• How to return from Firefox 5 to Firefox 4 | Firefox Support Forum | Firefox Help
  https://support.mozilla.com/en-US/questions/837464

Locate files with the name

places.sqlite places.sqlite-shm places.sqlite-wal

on your profile. Close firefox, make a backup of the files and start firefox. I had the same problem and after deleting the files, firefox came up with all my bookmarks, back button was working again and a lot of other misbehaviour was fixed. So give it a try, but keep a copy of the files, just in case ...