Google acquired On2 in February of 2010. In August of 2010, the MPEG-LA "announced that H.264 encoded internet video that is free to end users will never be charged royalties." (http://en.wikipedia.org/wiki/H.264/MPEG-4_AVC#Patent_licensi...)

So... yeah, I'd say it profoundly affected the MPEG-LA's behavior. If Google didn't have an easy out with (what would become) WebM, their Youtube licensing would be much more expensive.

Whether that patent promise has any bearing on the Mozilla decision is a more complicated issue. People have strong opinions on both sides.