Firefox bug - Unrecognized storage name 'null' in about:cache URL
Opening about:cache does not show any entries. The reason seems to be that Private is unchecked. Doing that and trying to update the cache is resulting in an error:
Unrecognized storage name 'null' in about:cache URL
This is actually a Mozilla bug as this happens in a vanilla Firefox as well. But now clicking on Back to overview does give us the cache entries as Private is checked.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information