Hope this is in the right spot - if not somebody can move it - but the test version of Fire Fox 3 is available more for website developers to experiment with according to the PC pro website this afternoon
Older versions of Windows ME and before apparently won't be supported.
see this link for the story
Mozilla warns that Gecko 1.9. Gran Paradiso Alpha 1 is available for testing purposes only and is intended for Web application developers and Mozilla's testing community. With that in mind, the software can be downloaded from mozilla.org/projects/firefox/3.0a1/releasenotes.