Go to C:\Documents and Settings\_____(your name)\Application Data\Mozilla\Firefox\Profiles.
Generally, there will be a folder in here that is named just random letters, followed by ".default."
If there is only one, go in there and then look for a "bookmarkbackups" folder. Look at these files. If one of them contains the bookmarks you want, import them into Firefox by going to Bookmarks -> Manage Bookmarks and then File -> Import (from file).
If you have multiple profile folders, look in all of them for your bookmarks. In this case, you should also look at the bookmarks.html file in each profile folder.
