When you deleted Firefox 4 beta did you choose the option to "Remove my Firefox personal data and customizations"? If you did it will have deleted all of your Firefox user data including your bookmarks. System restore will not help in recovering the data. If you did not choose that option, the [[lost bookmarks]] article may help in recovering them. If you did choose that option, do you make your own backups? If you do you can recover the data from the backups. You can restore the places.sqlite file from the backup and place it in your current [[profiles|profile folder]]. If you do not create your own backups, you can try using a file recovery program such as [http://www.piriform.com/recuva Recuva]. See if you can recover a file with a name in the format bookmarks-yyyy-mm-dd.json, these files would have been inside the bookmarkbackups folder in side the [[profiles|profile folder]]. If you manage to recover/undelete one of those files, you can import the bookmarks using the restore option as shown in the [[Backing up and restoring bookmarks]] article.