Sadly, mozex isn't working anymore for Firefox 3.0. I was looking at hacking it to make it work, and make it work better with Emacs, but then I found:
Unfortunately, this didn't seem to work with my emacsclient setup, until I found:
Emacs as default View Page Source / Greasemonkey editor in Firefox on Windows which tells us that Firefox will only accept a path as an argument to view_source.editor.path, no flags are allowed.
So, I just editted my "firefox-emacsclient" script to look like this:
/usr/sness/emacs/bin/emacsclient -c -n -s sness $1
and I can get beautiful source code viewed in Emacs. Nice.