I think there is probably a lot of work to do to fully pry the .mozilla folder apart. For a long time they've simply shipped everything in that folder and rolled with it. Making decisions on what is actually cache and what is user config vs "application data" is probably going to be harder than splitting the folder.
That's true, but they've already done it for macOS... ~/Library/Application Support/Firefox/ (for both the config and non-config data) versus ~/Library/Caches/Firefox/ (for cached data that can always be deleted)
Oh, I hadn't even _thought_ of that. Yeah, that's going to be a fun debate. Realistically, extensions shouldn't care about the folder structure of other parts of the profile, but I also know that there is a _lot_ of history there.
Requiring a mozconfig file shows that the code base has failed to transition to cmake or meson/ninja (directly; there is some python wrapper which may help here but I refer to the primary configuration). Mozilla gave up on Firefox a long time ago already.
