Do you have Sync turned on? (Firefox settings -> Sync) If you do, turn off syncing bookmarks and try deleting the tags.

How do I turn it off? See attachment.

Thanks

Open "Manage sync..." and unselect Bookmarks, then Save Changes

Did what you said still shows this

Assuming you tried deleting the tags again *after* turning off sync for bookmarks...

Could you please read https://support.mozilla.org/en-US/questions/1276050#answer-1277074

You can skip the first block of text since it isn't working for you.