He's a short version: AAPL and MSFT have a stake in MPEG-LA owner of the proprietary H.264, for now they lets us use it without paying royalties, Mozilla and other smaller players can't support because it's proprietary. One day MPEG-LA will ask for it’s money and we’ll all be royally fucked, it happen before with GIF, so please support any existing or to introduced open source alternatives.