Plug-ins were brought up by someone else already. I have to say that on Windows PCs and laptops under my control, Firefox will stay as number one browser for a while due to the plug-ins. I have Adblock Plus and Adblock Filterset.G to block a lot of cruft. Flashblock to stop the blink tags that pass for ads these days. Session Manager so that when I shutdown the browser I can resume with all the tabs I had when I last used it and it helps, but doesn't completely solve the losses from a browser crash. IE tab if something doesn't render as expected in Firefox (I barely use this one anymore).
Firefox 2 and 3 will stay prime for me, butI will keep an eye on Chrome and I hope Mozilla and others do too.
Brian Kelsay