I think the specifics matter here and I'm not in any way versed well enough in law to know so correct me if if I'm wrong but:
Mozilla could absolutely use and distribute x265 but that would also require them to have a license for the patents required for HVEC and would also require the users to have a license (which is of course unrealistic).
The only reason firefox can play h264 is because Cisco paid for an unlimited license with subdistribution and assigned it to mozilla.
So as h265/HVEC is concerned the problem for mozilla is that even if someone wanted to pay the bill like cisco did they would not be able to get a straight answer who and what they needed to pay for a license, right?