~/.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.
From what I understand, when the directory was added VR, MR, and 3d printing were just becoming more main stream. It is easy to see why some at Microsoft believed that an easy to use modeling applications like Microsoft 3d Builder would be as essential as Microsoft Paint. With hindsight, we can see that 3d printing is still too complicated and not useful enough for the average computer user and most VR users are either content to use models from others or to use more complicated tools like Blender.
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.