OS support is a little more nuanced than that.

The latest version of Firefox for OS X requires at least version 10.4 (released 2005). Ditto for Opera.

Google Chrome requires at least 10.5 (released 2007), and doesn't support PPC (the transition to Intel was `completed' in 2007).

Windows Vista went to retail in 2007.

Firefox, Opera and Chrome all still work on Windows XP though.