It's not nearly as simply as just dropping ffmpeg in instead of Xiph though. As
you said, they'd have to pay for the license themselves for one. Outsourcing
decoding to system APIs get's around that.
Also I'm not sure they could pay MPEG-LA and still maintain the re-
distribution rights that the MPL gives you. You would need to pay the MPEG-
LA to distribute any modified version of the Gecko engine.