I haven’t installed any new add-ons in quite a while - actually since I bought my current PC in mid '18.
Just now (using Win10 with Firefox 66.0, 64 bit) I tried to install a new one (and then, in case the first one was incompatible, several more), and all I could see was “only on Firefox - download Firefox now”.
If I follow that link, I eventually see “You already have Firefox”.
In case this should now be mandatory, I logged in; no change.
Can someone please help me in my ignorance?