I have had to reinstall my OS a few times, and this has left me with several "phantom" computers, and I don't know how to remove them.
When I go to the "Tabs from other devices" page, I see a bunch of "installations" that I don't have anymore. How do I remove them?
This question seemed like exactly what I needed, but the answer simply didn't work. He said that you can deactivate devices via account.services.mozilla.com, but this is either no longer true, or only works for Mobile installations.