Isolate font cache to first party
https://bugzilla.mozilla.org/show_bug.cgi?id=1647732 implemented top-level site partitioning for the font cache and solved the FPI part as well.
We should try to backport the patches for desktop. On mobile we'll get the feature with 10.0.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information