Firefox keeps backups of your bookmarks automatically. Please follow the steps in the following MozillaZine knowledge base:Restoring bookmarks in Firefox 3
Starting in Firefox 3, you can replace your current bookmarks with a bookmark backup file using the "Restore" feature of the Bookmarks Manager ("Bookmarks -> Organize Bookmarks -> Import and Backup -> Restore...
"). You can select one of the dated backups that Firefox automatically creates or you can use the "Choose File..." option to select your own backup file. (If you see the error, Unable to process the backup file, read this article.)
The automatic bookmark backups that Firefox creates are stored in the bookmarkbackups
folder, located in the Firefox profile folder. You can also create your own bookmark backups using the "Import and Backup -> Backup...
" feature. All bookmark backup files are stored in in JSON format. Any bookmarks saved in HTML format (such as exported bookmarks) must be imported (see above) rather than restored. 
Note: You can also restore your bookmarks by exiting Firefox and removing the places.sqlite file from the Firefox profile folder. Firefox will rebuild the Places database from the latest JSON backup or, if no JSON file is found, from a bookmarks.html file, as detailed here; however, you will lose your browsing history. Source - http://kb.mozillazine.org/Lost_bookmarks
Hope that helps,
Edited by Joshuacat, 12 April 2009 - 05:31 PM.