Your use of Firefox must follow Mozilla’s Acceptable Use Policy, and you agree that you will not use Firefox to infringe anyone’s rights or violate any applicable laws or regulations.
Acceptable Use Policy links to https://www.mozilla.org/about/legal/acceptable-use/ which says "You may not use any of Mozilla’s services to[...]Upload, download, transmit, display, or grant access to content that includes graphic depictions of sexuality or violence"
It's against the Terms of Use to use Firefox to... watch porn?
Either their legal team made a mistake, in which case they should correct it and issue an apology ASAP, or they really do intend to own you, in which case I recommend switching to an alternative browser which is only a browser, like Dillo, Ladybird, or Netsurf.