It's unquestionably illegal to distribute. As in, if Mozilla (or anyone else hit by the DMCA) were to add it back to the store, they would be acting illegally.
Why do people keep repeating this BS. The DMCA doesn't grant random lawyers the ability to make things illegal. All does give platforms the ability to get out of lawsuits by pre-emptively taking down the things mentioned in the notification but that doesn't make not doing that illegal.