Comment 42 for bug 1551171

It is still relevant; see for example https://bugzilla.mozilla.org/show_bug.cgi?id=1304650. https://wiki.debian.org/MIME has succinct info about the cache file, and https://bugs.launchpad.net/ubuntu/+source/gimp/+bug/574342/comments/22 describes a workaround.

You could argue that this is a desktop environment bug, but from the user perspective "the right thing" happens when they open the downloaded item via their file browser of choice, and changing the default file association in their desktop environment settings tool doesn't influence Firefox behavior. Firefox is the outlier, and ought to be using the same handler application that xdg-mime would return.