Sorry but from what I've seen in the API I cannot unload the button. Another option would be to use the pageAction https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/pageAction. But by doing this you cannot access the options unless there are duplicates tabs or if you go to the "Add-ons Manager" tab (about:addons). It seems less convenient to use.
