It seems the preview for IE10 release by Microsoft last week will only work on Windows 7 (32-bit or 64-bit) and the future Windows 8. Microsoft has already shut-out Windows XP users from IE9, so now they are following suit with Vista and the future IE10. The good news here is Firefox 5 (due out in late June) will continue to support Windows Vista and (far as I know) Windows XP.
Source: Seattle PI: Microsoft Blog
I think the listed hardware requirements on firefox 4 are inaccurate. It says it requires a pentium 4 with 512 mb of ram. I have it running very fast, faster that 3.6 on a pentium 3 with 256mb of ram. 3.6 recommended system requirements were only 128 mb of ram. I figured the recommended system requirements would double between 3.6 and 4, but not quadruple. If I had read them before hand I would not have bothered installing firefox 4. I think you are turning off many people by making them so high, and that you did not actually test the minimum system specs but just guessed. You realize that the system specs may be dependent partially on the system specs of the os. So vista and windows 7 will run much higher than xp and 2000, or win9x with kernelx.
I am running ubuntu with the light lxde desktop so my system reqirements are lower than someone running it with the default gnome desktop.