Files with the extension .FRI are backup files of the 'Mozilla Firefox' bookmark feature. The .FRI file contains backup data regarding the locations of all your selected bookmarks so that should your bookmark file become corrupted users are still able to reinstall their bookmarks.

.FRI files are created every time that Firefox is closed down and will be saved as a double extender of your original bookmark file i.e. BOOKMARK.HTML.FRI (original file = BOOKMARK.HTML)

.FRI backup files are by default found within the backups folder within Firefox's profile folder.

To restore an old .FRI backup file in place of a corrupt bookmarks file: close Firefox, find your backups folder (as above by default), locate the sub-folder from yesterday or a previous day and copy the .FRI into your profile folder in place of the corrupted file, restart Firefox.