I do not think that is possible, unless you have backups.

However if you have not deleted your History and have visited the site recently you may find it in your history. (Did you for instance open the bookmarked site and delete by clicking on the bookmark edit star icon).

The other possibility is that it is still in either any manual backups you have made.
but I guess you would not be asking if you had backups.

Or as long as you did not delete the backups Firefox makes (or your profile) then you may attempt to recover from them. Heed the warning to make backups before and after each restore attempt because the restores overwrite whatever you already have.

It is good practice to make backups manually on a regular basis, if you do not have any other automated backup system.

Yet another possibility is if you use sync you may be able to get it back from the other device or the sync server.

See

• backing up and restoring bookmarks
• what is firefox sync?

(edited typos including fixing link)

If you have accidentally removed bookmarks then use "Organize > Undo" in the bookmarks manager (Bookmarks > Show All Bookmarks) to get them back.
The Organize button is the first of the three buttons on the toolbar in the Library (Bookmarks Manager).
That only works if you haven't closed Firefox.
If it didn't work then you can restore the bookmarks from a JSON backup.
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