Ok, it took me a while to figure it out, it’s something to do with Firefox pulling the image from its cache. You need to whitelist the wyciwyg-scheme scope. wyciwyg-scheme really means that Firefox is pulling the image from a sort of wyciwyg: URI (its cache I believe), and since that scope is not whitelisted by default, this was breaking pulling from the cache – that is about as much as I understand for now.
So anyway, create this rule in My rules, then persist, issue should be gone for good. I will add it to uMatrix’s default ruleset.
matrix-off: wyciwyg-scheme true
I think wyciwyg means “what you cache is what you get”.