Let macos users choose a Firefox icon, and follow system appearance

[a266]

The ios app already lets you pick a different icon but mac users get nothing like it. I’d like Firefox on macos to offer too a list of alternate icons in settings.

Right now the icon stays the same whether the system is in light or dark mode, so a bright icon ends up sitting in a dark dock, which looks wrong to anyone who switched to dark mode on purpose. Firefox could also read the system appearance and swap between a light and a dark variant automatically.

A "follow system appearance" toggle would cover people who want the automatic swap and a manual choice would still win for anyone who wants a fixed icon.

It’s mostly a preferences and packaging change and it would make Firefox on mac feel like something nice rather than a generic app sitting in our dock.