IE can't really run around updating itself every 5 to 6 days, IE is used in environments where having a long term browser that works is a must, that is why you still see people using IE6 in corporate environments because it works for them, if MS went around having IE 6.1/2/3/4/5/6/6.1/6.2/6.3 etc all in the spate of a few months then the corporate bean counters would go nuts. Update IE6.1 to 6.1.5 and X breaks, upgrade to 6.1.6 and Y breaks, .7 fixes X and Y but breaks Z and A and also B, get my drift?
330m users or 330m downloads? Big difference, I'm sure lots of people have downloaded FF and thought "what a pile of bloated tosh!" and deleted it, I know I did (chrome is super sexy, all hail google our master overlords!). Still at least its better than Safari.
Though lets face it, how hard was it really for FF to become the 2nd most popular browser? Not exactly much competition in the browser marketspace at the time was there........