1. Like others have commented, I have not experienced any performance issues with Firefox. I'm still using V3, and often have 4-5 browser windows with 5-10 tabs each open at any given time. If memory usage gets high (like after a week or so), I just close and reopen FF...takes 1 minute and is a cinch because FF has had the functionality of restoring the complete browser state upon restart for a long time (long before IE).
2. Something I didn't see mentioned yet: Firefox has (in my opinion) done the best job of adhering to W3C standards and rendering web content properly.
3. I tried giving Chrome a test run about 4 months ago, and was surprised at the lack of functionality and ability to customize the browser's interface.
4. There are a ton of great, extremely useful Add-ons available for FF. The author mentioned Xmarks (which I also use), but there are just too many more to mention. I didn't see that many available for Chrome (but, admittedly, I didn't search too hard after being disappointed with #3 above).
5. As a long-time advocate of open-source and an open Internet, I appreciate (and contribute to) Mozilla's mission which supports these objectives. They make a great E-mail program, too! (Thunderbird).