My post SharePoint Accessibility – Is MOSS accessible? refers to Level 1 and Level 2 browsers and the support provided.
Microsoft have now published a TechNet article on this subject.
Level 1 Browsers
IE 6.x (32bit) IE 7.x (32bit) – Where is the 64bit support?
Level 2 Browsers
FireFox 1.5, Mozzila 1.7, Netscape Navigator 7.2 and 8.1, Safari 2.0