It was never the intention of this add-on to allow people to switch containers through the tab context menu. At no point in its development this was ever considered as part of its roadmap. The functionality was always limited to just the one button on the menu bar, which is what the add-on aimed for, and which it has continuously been able to do without issue. The development of this add-on is not "inactive" but rather "finished for the time being."

I'm not responsible for people's wishes for this add-on to do more than it ever intended to do, and users of this add-on aren't entitled to demand more features when they were never promised or considered. I'm only responsible for making this add-on do what it purportedly aims to do, which it already does.

More importantly, the official "Multi-Account Containers" add-on by Mozilla, and further updates to the browser itself, have made this issue — and my add-on itself — entirely obsolete. You can at this point right-click on a tab and find a menu which allows you to switch containers. You don't need anything more than the base Firefox browser and Mozilla's official add-on, which you'd need anyway since that's the only way to set containers up.