You can open the Bookmarks Manager (Library) and expand the Tags folder in the left panel and select the tag you want to remove. This will show all bookmarks with this tag in the right panel. You can select all the bookmarks in the right panel via Ctrl+A and remove all those bookmarks. Once there are no bookmarks left with this tag then Firefox should remove this tag from the left panel. You can repeat this by selecting another tag in the left panel. See also: *https://support.mozilla.org/en-US/kb/categorizing-bookmarks-make-them-easy-to-find