Firefox stores backup copies of prior bookmark files, and these files can accumulate, taking up a fair amount of space. I found 129 files, the most recent being 300 KB. Deleting all the backup files saved about 40 MB in disk space.
These files are stored in your user's /Library/Application Support/Firefox/Profiles/bookmarkbackups folder. Delete the ones you don't need and you may save a lot of space.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060424151548561