http://www.techspot.com/news/44076-google-to-drop-support-for-ie7-firefox-35-safari-3.html
Each time a new version is released, the company will begin supporting the update and stop supporting the third-oldest version.
As of August 1, 2011, Google will discontinue support for the following browsers and their predecessors: Internet Explorer 7, Firefox 3.5, and Safari 3. In these older browsers you may have trouble using certain features in Gmail, Google Calendar, Google Talk, Google Docs, and Google Sites. The company also said that eventually these apps may stop working entirely in older browsers.The company specifically lists four browsers to choose from, in a very specific (alphabetical) order: Chrome, Firefox, Internet Explorer, and Safari. Opera is notably missing from the list.