This is just a little experiment to see how easy it would be to switch some of these messages to use moz-message-bar. If we think this works well then I'll file a handful of bugs to tackle similar UI, all of which should be pretty contributor friendly (I can point to this patch for reference).

There are a couple of changes to moz-message-bar to make the conversion easier - these are actually copied from work in D189872 which I'll likely land first (sometime early next year, so no rush on this).