~/.mozilla/firefox/<profile> is a mish-mash of data, config, and cache. It's not simple to unravel that.
Beyond that, it would be a breaking change, and that requires more caution.
The fun part is, at least since Win95, games were supposed to store their data in the My documents directory. The only game I remember actually going with this was Max Payne, everyone else was just dumping everything in the directory with the exe.
Then Vista came out and those who cared, were scrambling to update their games. The rest just went - shrug eh, run in compatibility mode.
I thought this was the best thing they did. Each user would have their documents (let call it your stuff) separate from another user.
If you share a PC with someone then you would know how this was a good thing.
Even if you don’t share your PC with anyone, you don’t want your nephew to miss up your saves when he comes in the summer and his mom wants you to allow him to play.
Windows's dedicated Saved Gamed folder is within the same user-specific directories that Documents and AppData are in, and would still allow for game saves to be user-specific.
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
I think moving the folder under ~/.local before splitting the cache folders out is a bad idea. Many people will have specific backup or sync solutions in place that want to include config, recreate data, and exclude cache, so the XDG spec has separate locations for them.
I know, it's not a complete solution, but it would at least serve as a stop gap to clean the mess out of the home folder, before the actually compliant implementation is made, XDG_DATA_HOME should always be saved as it contains the user generated data of an app (that isn't documents)