It will pretty soon be possible to turn off top tabs in Firefox without this kind of userChrome jiggery-pokery. The sidebar API has been updated in Firefox Nigh

It will pretty soon be possible to turn off top tabs in Firefox without this kind of userChrome jiggery-pokery. The sidebar API has been updated in Firefox Nightly 131, and it provides a side tab bar, and the ability to disable the top tab bar. Existing sidebar tab extensions will be able to update to this API.