Which version of Firefox (go to help/about)? The latest versions (1.5x and later?) keep a daily backup for just this reason. You'll need to use My Computer to navigate through "c:\documents and settings\your username\application data\mozilla\firefox\profiles\some weird folder name\bookmarkbackups". You'd better do this before a full week ends, or you'll for sure lose the bookmarks.

In order to get this far, you may have to change a folder setting in My Computer. Go to Tools/Folder Options/View. Then click on "show hidden files and folders". Click on apply to all folders, unless you want to do this for each hidden folder you navigate into.