Google seems to be concerned with improving the user experience in Gmail and sometimes upgrading your browser can really speed up web applications like Gmail, especially when you are using Internet Explorer.
I happened to open Gmail in Internet Explorer 6 and I noticed a message that invited me to "get faster Gmail".
Google didn't recommend to install Firefox or Chrome, like it did in other cases, but it suggested to install IE7. "We noticed that your current browser, Internet Explorer 6, is out of date. In order to improve your Gmail experience, we suggest that you upgrade your browser to a faster Gmail supported browser that works on Windows."
Upgrading to Internet Explorer 7 is a good tip, but I was surprised to see that Google sends users to a customized version of IE7 that includes Google Toolbar and changes your homepage to google.com. Downloading additional toolbars should be optional and it has nothing to do with Gmail's performance. What's more, the customized package includes outdated versions of IE7 and Google Toolbar from 2006.
A similar page from Gmail's help center explains that you can "make Gmail run an average of twice as fast" by installing IE7, Firefox 3 or Chrome. Google links to the official homepage of Mozilla Firefox, instead of recommending a customized version.