It's been taken down by DMCA copyright strikes on every western platform that could distribute it. That's why you can't find it on the Firefox extension store anymore
None of that means it’s “illegal”. You apparently think something being removed to reduce legal risk means it’s illegal, but that doesn’t follow.
It's illegal to distribute. Unambiguously. That's how DMCA works. If Mozilla were to add it back to the store, they would be acting illegally.