I think; If you have not closed Firefox, you can undelete what was done.

These can't get your data back, but will help in the future.

These add-ons can be a great help by backing up and restoring Firefox

FEBE (Firefox Environment Backup Extension) {web link} FEBE allows you to quickly and easily backup your Firefox extensions, history, passwords, and more. In fact, it goes beyond just backing up -- It will actually rebuild your saved files individually into installable .xpi files. It will also make backup of files that you choose.

OPIE {web link} Import/Export extension preferences

If you haven't closed Firefox yet then see if you can use "Organize > Undo" in the bookmarks manager (Bookmarks > Show All Bookmarks) to undo this action and restore the bookmark(s). The Organize button is the first of the three buttons next to the Back and Forward buttons on the toolbar in the Library (Bookmarks Manager). If it didn't work then you can restore the bookmarks from a JSON backup in the bookmarkbackups folder.

• Bookmarks > Show All Bookmarks > Import & Backup > Restore

This will replace all current bookmarks and you lose bookmarks that were added since that backup was created. See:

• http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox