I'm a big fan of Gmail - especially when it comes to technical mailing lists and the contextual advertising it provides. I quite regularly use the suggested links for product research.
Up until recently, I'd been using Gmail inside Internet Explorer 7. Around the time of the new Gmail interface, Gmail started getting slower and IE7's memory profile started getting bigger. IE7 also got a lot less stable, so much so that I needed to not only install IE7Pro for session crash protection, but I had to launch a separate IE7 process just for Gmail.
I finally got sick of the instability and slowness, so I thought I'd give Firefox a crack at it. Not only is Gmail more responsive under Firefox, but Firefox is using a lot less memory than the equivalent IE7 process and it doesn't keep growing like IE7 does.
I get the distinct impression that there are quite a few memory leak and garbage collection problems in IE7, which makes it a problem in a Web 2.0 world, or for browser power users. It would be nice to see Microsoft address this problem in current and future releases of their browser.