If you mean add Switch Container in the right-click context menu for tabs, that's outside the scope of this add-on.

The API only allows me to pick one icon for the add-on, so I went with something that worked well on the default theme (which most people use) AND dark themes (which I use). I'm waiting for Mozilla to allow add-ons to have icons follow the user theme's color scheme, but I've yet to see this be allowed officially. Otherwise, while API does allow me to give people the option to pick a color for the icon, I find this is not an elegant solution to this problem.

Fixed in next version 0.7: Pinned tabs remain pinned after changing container. However, due to API limitations, tabs have to be positioned back into their original order.

Why? This has to do with how the add-on actually changes a tab's container, which is, by duplicating the current tab into a different container tab and then removing the original—which is the only way to do this using the API, and pretty much an unintended usage; a hack, if you will.

Since the resulting container tab is pretty much just a new pinned tab, it will always be added to the right of the rest of the pinned tabs, and there's no way to control its position thru the API. Therefore, only the user can reorder pinned tabs.

Additionally: An add-on like Multifox had considerably more access to the browser than a WebExt add-on could ever have, and therefore not all features can be exactly 1:1. In any case, it has never been my intention to replace Multifox, but to allow exactly this one extended functionality to the official Container Tabs add-on/feature by Mozilla.

Thanks for your review! Regarding it being a pageaction: It used to be in the address bar at one point, however there's a reason why it's not a pageaction now. I quote:

"Page actions are for actions that are only relevant to particular pages. If your icon should always be available, use a browser action instead."

So I opted to follow the proper guideline as established by the Firefox developers.