This add-on makes dealing with containers much easier, but it requires a lot of polishing.

I just found three bugs:

1) Customisation mode cannot be activated, because the add-on tries to "contain" it, what causes it to break (the add-on should check if it's the customisation tab that's being opened and avoid trying to "contain" it). Other built-in Pages work fine (about:config, about:preferences and so on). The bug only shows when the customisation is activated on a window with containers.

2) Control+N or File -> New Window opens a new container tab, instead of a new window! Once again, the bug only shows up when the commands are used on a window with containers. When done on a window without ANY containers, a new window is normally created (it doesn't matter if the active tab is "uncontained"). If, for instance, no window is uncontained, the is no way to create new windows (dragging a tab out of its window, luckily, still work, so it's possible to "produce" new "uncontained" windows by creating an empty tab, which will be contained, drag it out to create a new window, then switch the container of that new tab with Switch Container Plus, which is currently the only add-on able to change containers of empty tabs). Too much trouble just to create a window, but still possible.

3) When recovering from a crash, ALL tabs on ALL windows are "contained" (kind of randomly, depending on the first contained tab Firefox restores (it doesn't have to load it, just its presence in any window is enough to "contaminate" all other tabs). It's, in a word, disastrous, as all tabs change their containers (even uncontained ones), messing with logged in accounts, changing site preferences and whatnot.

Currently, this add-on is unusable (yes, it works fine enough while everything is running smooth, but then it may mess up everything in the event of a crash or restart).