Switch Container per MarsCat

Switch Containers is a WebExt-based Firefox add-on that allows users to switch the current tab's container on the fly, and revert a container tab back to a regular tab.

The icon theming can be toggled between rainbow and gray. Right-click on the extension's icon to change the theme.

IMPORTANT: There are security concerns involved when changing a tab between containers. Before installing, please read this article on the matter: https://github.com/mozilla/testpilot-containers/wiki/Moving-between-containers