First try and report this bug to mozilla.
Then go to:
C:\Documents and Settings\{Log In Name}\Application Data\Mozilla\Firefox\Profiles\{Random Stuff}
You might have to show hidden folders. Anyway, when you get here you need to look if something along the lines of bookmarks.bak If this file exists and it's more than 0 KBs, then go in to Firefox, go to Bookmarks > Manage Booksmarks > File > Import, Select from file, and open the file you just found.