I can't get this to work in 2019 on the latest version of macOS. (For one thing, macs don't technically *have* an alt key.)

Great!

Can you file a bug asking for that kind of API event?

I saw similiar things requested...check this out:
https://bugzilla.mozilla.org/show_bug.cgi?id=1232178

EDITED 05.09.2017:
I followed your link and saw it was marked as resolved.

As I see in the 'linked' bug, they won't implement the functionality. They see WebExt have to deal with the web, not the browser chrome :\

This webextensions thing is getting tiresome...almost nothing will work (I mean extensions, from before)

Good, but need to Pin/Unpin by Mouse Double Click on Tab. If you can to do this, It will be very usefull.

(re: Thank you, but Right click menu on tab is exist in FF... I'm looking for an alternative for an addons that will not support WebExtension eg. Tab Mix Plus or Find Bar Tweak or Panic Button and so on...)

small, fast and useful

It does what it's supposed to do and makes managing my tabs easier.

I use this not only to pin tabs, but also to move them. Invoking the shortcut twice moves a tab to the front of the list.