Mozilla has not quite fixed the security hiccups with URI protocol handling in Firefox.
According to Billy Rios and Nate McFeters, the two security researchers behind the exposure of protocol abuse in popular Web browsers, Firefox is still vulnerable to a remote command injection flaw that could allow hackers to launch executable code on Windows machines.
Nate and I have discovered a way to "...exploit a common handler with a single unexpected URI..." Once again, these URI payloads can be passed by the mailto, nntp, news, and snews URIs, allowing us to pass the payload without any user interaction. So, it seems that although the conditions which allowed for remote command execution in Firefox 188.8.131.52 have been addressed with a security patch, the underlying file type handling issues which are truly the heart of the issue have NOT been addressed.
Rios said Mozilla was contacted "a while ago" about this issue and has promised a more comprehensive patch.