-> press CTRL + SHIFT + B to open the Bookmarks Library

-> Now right-click on a Bookmarks folder and click Delete

Delete all your Bookmarks folders and links (as mentioned in the previous step) except these (these are Firefox's Default Bookmarks):

• Most Visited (folder)
• Mozilla Firefox (folder)
• Recently Bookmarked (folder)
• Latest Headlines

Check and tell if its working.

Deleting Bookmarks

• https://support.mozilla.com/en-US/kb/Deleting%20Bookmarks

You can also remove all bookmarks via the Save mode start window.

• https://support.mozilla.com/kb/Safe+Mode
• Help > Restart with Add-ons Disabled

Another possibility is the delete the files places.* in the Firefox profile folder and all .json backups in the bookmarkbackups folder.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Bookmarks