If you haven't closed the browser, you can open the bookmark manager (ctl+shft+o) then the menu at the top > Organise > Undo.

Otherwise, use that menu to restore the backup preceding the deletion. > Import and backup > restore : select the backup you want.

^: Note that Firefox keeps a full undo stack of changes to bookmarks for the current session.

So if you haven't closed Firefox, you can use "Organize -> Undo" (Ctrl+Z) until you see all deleted bookmarks reappear. Of course you lose changes made after you removed those bookmarks, so if necessary make a note of the recent bookmarks ("3-bar" Firefox menu button drop-down list -> Bookmarks).