Found the way to test Comet on Ubuntu - use Firefox 3, and Seamonkey.
Seamonkey seems to support all the necessary functions for Comet (and
it's based on Gecko, so you shouldn't see too much weirdness).
Seamonkey is in the World Wide Web universe category, as
seamonkey-browser. Installable with package manager out of the box.
Jim
P.S. I haven't found Opera on Ubuntu to be useful at all in Comet
testing so far - too much weird behavior.
Jim