There is one main reason why I don’t use Firefox.
98% of my “surfing” or “browsing” time happens in Outlook 2003, where Newsgator delivers my RSS feeds. The reading pane in Outlook is built on Internet Explorer. So I’m already hitting IE’s cache, security settings, admuncher settings, page behavior. Now if I were to click on the link to open a particular post in the full browser (which I do all the time) all that stuff is already there and I know what to expect. If I had Firefox set as my default it would have to go and re-download all that content and load up a second cache which is wasteful and slow. Plus, if the page doesn’t load like I expect it to, I won’t know if the problem is firefox specific or not.
Really though I’m glad that they are pushing Microsoft to build IE 7, but the whole business of getting so passionate about it is ridiculous. In the long run a lot of good will have come from doing all that work, but really the actual product is a bit of a distraction for Windows users.