The Firefox bookmark is at
you can edit them manually a text editor.
It's a JSON file. Open that, edit it, save.
Now, goto Firefox, open the bookmark manager 【Ctrl+⇧ Shift+o】, then menu 〖Import and Backup ▸ Restore ▸ Choose File…〗, then select the file you just edited. To show hidden file, press 【Ctrl+h】.