While using fully updated Firefox and logged in, I cannot download any addon because I am only asked to Download Firefox.
My browser is 65.0.2 (64-bit), up-to-date. When attempting to download/install a new add-on, I am provided no option for doing so. The only available button for download is "Only with Firefox, Get Firefox Now". This is the same for every add-on. I have tried using Firefox in Safe Mode with all add-ons off and it still did not work.
You get that message if you were not using Firefox at the time. If you were, check out this artical: