Sorry I don't know how to find the right place on github to write sth to the author.
You asked: "shouldn't it be handled by a separate extension? (like, for example, Facebook Container: https://addons.mozilla.org/en-US/firefox/addon/facebook-container/)"
No, that's totally different addon. I don't use fb.
I don't know how to find a similar link for you, yet to "Firefox multi acccount containers" addon, but you surely know that official addon from mozilla itself?
Pl try using your plugin on such an install.
It won't work if you use Pinterest in a "pinterest" container, because your plugin then opens pinterest in the standard container - which doesn't even know that I am logged into pinterest LOL
Maybe you can simply make your plugin open pinterest in SAME container where your plugin gets clicked from?