A pretty font selection GUI doesn't seem to be possible, so I decided to just let the user specify the exact value, since it's just a CSS `font` property (except for the size part, which my addon has to set automatically).
I personally use `"fira sans", sans-serif`.

As for the flickering: It does go away if you use the 'Global' option, which would make it so that the number is the total tab count for all windows. If you don't want that there isn't any other way I know of to solve this. It bothers me to, but what can you do, it's an API restriction. But maybe I should just make 'Global' the default.

update: since firefox 62 the flickering should be gone because of the new windowId parameter for browserAction.setIcon()