Check in C:\Documents and Settings\*username*\Application Data\Mozilla\Firefox\Profiles\*profilename* (I'd check all the profiles if there is more than one). The file would be titled bookmarks.html, and there are also backup files titled bookmarks.bak which can be renamed to .html.
You can check if the file has the bookmarks you're looking for just by opening it.
Once you have the right bookmarks.html, go to firefox and choose Bookmarks > Organize Bookmarks > File > Import > From File > select the bookmarks.html file.