Maybe my perception of Firefox being too slow is outdated. After all, only recently they introduced support to SPDY (AKA HTTP 2) protocol, which makes many sites appear much faster (specially Google's).
phpclasses.org/blog/post/182-Making
...
Firefox 14 still seems sluggish to me, but may be just my impression.
As for developing mobile applications with HTML 5/JavaScript/CSS that is nothing new. That is the way Web OS worked.
Such development environment is already provided for mobile application development under Android, iOS, Windows Mobile, BlackBerry, etc. using Titanium Appcelerator or PhoneGap.
Actually, there are even a few components for those frameworks in JSClasses sites:
jsclasses.org/browse/class/37.html
jsclasses.org/browse/class/45.html
I do not have anything against Mozilla, quite on contrary, but I am afraid their mobile OS comes too late to the game to succeed, and so it may be a waste of financial resources.