I use Mozilla for mail and news as well; it's not exactly feature-filled (Pan is for porn), but having the same tool in Windows and Linux is helpful.

Mozilla does get a lot of mileage out of their small wins. I would switch if the only advantage were that I could middle-click to open a link in a new window/tab, and IE scrolls too little with the mouse scroll and too much with the keyboard. Further, if you put a low limit on font size in IE and the web page asks for a smaller font, it messes up rendering, making the feature pointless. All of these things would be enough to get me to switch. So from my perspective, any larger differences are irrelevant.

What the masses will do I do not know and I do not care. We're long past the point where proprietary solutions can enter the market successfully, so the only good thing that would come from my perspective with Mozilla getting more popular is that Microsoft won't be able to get away with their special breed of innovation.

Regarding PNGs not working properly in IE, if you're serving from PHP you can hack around the lack of alpha blending by producing the correct DirectX bullshit if the browser in the HTTP request matches:

http://msdn.microsoft.com/workshop/author/filter/reference/filters/alphaimageloader.asp

I don't have the PHP code that did this any more. You can be assured it was Satanic Certified.