I'm a big fan of ChatZilla, I've used it for years, and for a while I was a fairly active contributor.
These days I spend most of my time on a Mac, where the ChatZilla experience isn't as nice as it could be, due to the cmd+tab behaviour when running as an addon. When ChatZilla is a part of the brower, it's a pain to switch between it and other apps you may be using.
The following instructions are very rough, more of a brain-dump while this is fresh in my mind. I'll try and tidy them up at some point.
The solution, is to run ChatZilla as its own application, there are some instructions for doing this at http://chatzilla.rdmsoft.com/xulrunner/, but I couldn't get these to work for me. I appeared to be running into this bug https://bugzilla.mozilla.org/show_bug.cgi?id=699966
Undeterred, I figured there must be another option - Firefox since version 3.0 has provided an "-app" option, which can be pointed to an xulapp application.ini, and will launch it standalone. Happily, this works! But we're left with an application called "Firefox" and with the firefox icon.
The solution now is found on this superuser answer: http://superuser.com/questions/271678/how-do-i-pass-command-line-arguments-to... specifically, option 2.
I made a copy of Firefox.app, renamed it ChatZilla.app, edited the Info.plist, unpacked the .xulapp into the application's /Resources folder and created a little bash script that would execute ChatZilla.app/Contents/MacOS/firefox-bin with the appropriate -app switch. All references to firefox in the Info.plist need to be changed to ChatZilla, to ensure the OS doesn't get confused about which is the actual default browser.
The final step was then to use http://iconverticons.com/online/ to take the ChatZilla .ico and turn it into an icns file for my new application.