To indicate an important active state of an add-on one should be able to set it to be highlighted. This will color the icon blue until the state is resolved. (e.g. set Bookmark or finished Download) This is a function chrome does not provide. I suggest calling it: setHighlight = true/false
This is going to need UX. We have this kind of styling for tabs, but not for any toolbar buttons that I'm aware of.
I think this is something like the bookmark star when you've bookmarked a page, or the reading mode toggle, or (what I think is the best example to copy from) the downloads button after you've downloaded something…
Well, those are all buttons that use separate icons to indicate a different state. Add-ons can already do that fairly easily. I think Markus is suggesting we add a separate highlight state that changes the button's background, like the 'titlechanged' state for pinned tabs. I could be mistaken, though.
Blake was right with his description. I am talking about enabling add-ons to use the same highlight-state as we use with a bookmarked page and a finished download. And to better integrate add-ons with Firefox we would like add-ons to not be provides as an svg we color in the matching colors for the os (bug 1207597) and want to define some default states for that for add-on devs to use. -> This would provide an alternate way of providing icons for add-ons that would perfectly integrate with our icons.