The Acceptable Use Policy contains guidelines for services and guidelines for products. The Firefox TOS says “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.” The only part of the Acceptable Use Policy that pertains to products is “You also may not sell, resell, or duplicate any Mozilla product or service without written permission from Mozilla.” Mozilla has a separate TOS for their services.
Therefore, you can look at porn in FF as long as you don’t bundle FF in a Linux repo without their written permission, but you can’t look at porn when using their VPN.
You could read it, it’s pretty short.
Here’s what the AUP says about porn:
So yeah, in that sense it “says what it says about porn.” It’s just that “what it says about porn” is in a list of things you can’t use their services for and before the only mention of how to use their “product.”
Through their various agreements and terms Mozilla makes a clear distinction between products and services and has clear guidelines on how you can use them. When the TOS says “obey the AUP” and the AUP says “don’t use our services for porn and don’t sell our products or services” then viewing porn with their product is not a violation of their AUP and thus not a violation of their TOS.
Ultimately, however, the final decision would have to be resolved in court.