Is that even a bug? It's more like a unwanted feature hardly anyone uses and as such hardly anyone ends up caring about in either direction. On the other hand

Is that even a bug? It's more like a unwanted feature hardly anyone uses and as such hardly anyone ends up caring about in either direction. On the other hand changing the icon through JS is something people do care about (unread notification indicator) for example. I seen one or two cases from "still icon" -js-> "animated icon" -js-> "still icon2" which where ok (or did they just do a lot of js refresh??). I can't remember any side abusing changing or animated icons, tbh. So if I would manage Firefox I probably wouldn't fix it either because it's not worth the time investment needed, while on a project the size of a browser there are basically always relevant issues to fix. (Exception maybe if I have to touch the code in question anyway and remember the issue.) And the age in the end doesn't matter if there are always docents of way more relevant bugs, they always will get priority. It only would matter if I care about irrelevant stats.