Hi,
I've have just verified it to work with Firefox 3.5 beta IF you use
Firebug 1.4 (the alpha version, not an official release).
I have updated the version support at
https://addons.mozilla.org/en-US/firefox/addon/11905/
When that have propagated through (might take a few hours), you can
just install it from there.
Best regards,
Robert