The idea and implementation is actually very good, but the UX is killed by the very restricted set of container icons and colors. You should allow us to go wild with experimentation about use cases. The builtin icon and color set should be at least thrice as blig, but for "power users" you should allow to take anything from an emoji/color picker!

Edit: I understand that these limitations exist in the browser's container implementation, not in this add-on. Therefore I don't want to be hard handed with the add-on, and bumped the stars to four. Still, the add-on is not a third-party independent work but a part of the implementation of the feature, so it's not possible to judge it isolated from the allover implementation. Thus I won't give five.