I said to use firefox trunk not 2.x I also noticed firefox 2.x handles Vista worse than 1.5.0.x, this is why I did not mention or suggest using it. I will get the MSDN documentation about default browser on Vista if you really need proof you are getting it from the wrong place.
All your reported issues with default browser confusion etc are based around the fact the firefox versions you are using are not correctly setting themself as the default browser.
You can download a trunk build here:
http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/latest-trunk/
