At least according to the wikipedia page, it looks like the Mozilla Foundation brings in $300M a year in revenue. I still wouldn't say that $6M a year is trivial for them, but it certainly isn't an impossible bridge to cross. Up until now the Mozilla party line has been that they don't want to pay the patent fees for reasons of principle.

So it looks to me like there was another option: bite the bullet, pay the licensing fee and distribute their own blessed binaries (they still couldn't grant a license to people building from source), then feel free to take whatever position on WebRTC they thought was best.