Reminds me of when Windows made a designated folder for game saves.

And then every dev decided to keep placing saves in the Documents folder or AppData folder

This is one of the reasons I use Flatpak whenever I can. I've revoked all Flatpak apps the permission to access the root of my home directory via a global override, so anything they wanted to do in there (e. g. create folders, place malicious code in my ~/.bashrc, etc.), actually happens in ~/.var/app//.

Many rather treat standards as suggestions 😒.
Jokes aside, I have wondered what prevents them from doing it too, I guess they probably don't think it's important enough to really work out how to split up the files.
Then again, moving the whole folder to ~/.local/share/mozilla would have been decent enough as a temporary solution