As Google announced last year, they support the latest version of Google Chrome (which automatically updates whenever it detects that a new version of the browser is available) as well as the current and prior major release of Firefox, Internet Explorer and Safari on a rolling basis. Each time a new version of one of these browsers is released, they begin supporting the update and stop supporting the third-oldest version.
Internet Explorer 10 launches on 10/26/2012, and as a result, Google will discontinue support for Internet Explorer 8 shortly afterwards, on 11/15/2012. After this date users accessing Google Apps services using Internet Explorer 8 will see a message recommending that they upgrade their browser.
For more information:
http://gmailblog.blogspot.com/
http://support.google.com/a/
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