According to TG Daily:
Taking a page out of Apple’s book, Google is now urging Gmail users to drop Internet Explorer 6 (IE6) in favor of Firefox or Chrome that, according to the company, run the popular web-based email service “twice as fast.” Google also labels IE6 as an unsupported browser, meaning it fails to run some Gmail features.
I had read about Google doing this earlier, but didn’t think to add them. However a recent discussion on Slashdot (Google Tells Users To Drop IE6) had me reconsider.