Right now there is a bit left to be desired when it comes to lemmys accessibility features, but it’s a good idea to be mindful of the fact the fediverse and its platforms tends to have pretty universal accessibility features that will likely come to lemmy sooner rather than later
Technically inserting the ALT text into the alt attribute is exactly right. Screen readers will treat it correctly and browsers are free to render it if necessary. This is more a Firefox issue than a Lwmmy issue, in my opinion.
Maybe frontends should add the ALT text to te title attribute for good measure, but that’s often cut off and may confuse screen readers.