make sure firefox is closed.
goto start, run (or search using vista). copy/paste this
%appdata%\mozilla\firefox\profiles\
in here will one (or more) random named folders. check each one and inside should be a folder called
bookmarkbackups
open the latest file (they're dated) and check your bookmarks are there, rename it to bookmarks.html and copy it one level up so it sits in the randomly named folder. start firefox and it should be there.