Except that it might be in Mozilla's best interest to make it compatible with all the major browsers if they want this to see any sort of real adoption.
Why?
Firefox + Chrome make up the majority. In mindshare, a vast majority.
Mozilla's only interest is in advancing the web, and web technologies. Not catering to a competitor's old technology...
Because until IE reaches Opera levels of usage, there are a large number of sites that don't target programmers that won't be able to build on stuff like this, because they don't want to cut out a big chunk of their potential market.
Everyone is happy, except those who want goodies for free. ;)