If you have accidentally removed bookmarks then use "Organize > Undo" in the bookmarks manager (Bookmarks > Organize 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

Thank you for your reply. When I click Undo, all I see are my current Bookmarks listed. I don't see any of my deleted bookmarks. Am I missing a step? I don't want to do Restore because I've added a lot of bookmarks since I deleted the ones I want to restore. Thank you!

Once you've closed Firefox then you can't use Undo anymore to get back deleted bookmarks. In such a case only a restore will get them back. You can do the restore in a new profile folder and copy and paste the deleted bookmarks from the Library from that new profile to the Library in your current default profile.

You can use the Firefox Profile Manager to create a new profile.
You can create a desktop shortcut that opens Firefox with that profile and also add the -no-remote switch to the Target command line to make it possible to have both profiles open at the same time.

"C:\Program Files\Mozilla Firefox\firefox.exe" -no-remote -P "Default User"

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile