STR:

• Open a fresh Firefox, with a fresh profile
• Install the multi-container addon. https://addons.mozilla.org/firefox/addon/multi-account-containers/ (Keep the tab open on that amo url)
• Open a new tab
• In the address bar, type "% addon". It will show the url above (and about:addons if like me you ended up there from about:addons)
• Now open a tab in a new container, and open some page in it, say, Wikipedia.
• In the address bar, type "% addon". It will show nothing, and AIUI, that's expected.
• Now open a new tab, which will open in the original container. In the address bar, type "% addon". It will show nothing, which it should show the same as the first time.

I've had this happen when switching to existing tabs in the original container too, and it's not been entirely reliable to reproduce (that is, sometimes it works properly).

The above is with Firefox 66.

(In reply to Mike Hommey [:glandium] from comment #0)

• In the address bar, type "% addon". It will show nothing, and AIUI, that's expected.

Yes, the switch to tab currently is per container. IIRC there is a bug about adding a pref to allow making it global.

• Now open a new tab, which will open in the original container. In the address bar, type "% addon". It will show nothing, which it should show the same as the first time.

Yes, it sounds like a bug. The fact it's not possible to reproduce it constantly though makes it hard to debug it :/