Windows/Linux
Extract the XULRunner archive to a sensible location.
For example, C:\Program Files\ or /opt/.
Save the ChatZilla .zip file inside the XULRunner directory. Open a terminal window.
Note: Ensure you run the XULRunner commands as an administrator. For example, on Windows 7, right-click Command Prompt on the Start Menu and choose "Run as administrator"., or on Linux, use "sudo" before the XULRunner commands.
Switch to the XULRunner directory.
For example, enter cd C:\Program Files\XULRunner or cd /opt/xulrunner.
Register the XULRunner location by entering xulrunner --register-global.
Install ChatZilla by entering xulrunner --install-app chatzilla-0.9.88-xr.zip.
Note: You can delete the ChatZilla .zip file after this step.
Start ChatZilla, which is installed in Program Files or /usr/lib depending on your OS..
Mac OS X
Double-click the .dmg file to mount it, then double-click the installer .pkg file inside it. Follow the instructions to install XULRunner.
Open a terminal window and run /Library/Frameworks/XUL.framework/xulrunner-bin --install-app "<package>", where <package> is the full path to the ChatZilla .zip file.
Start ChatZilla in your Applications folder.
If you need to copy over settings or scripts, use the ChatZilla command /pref profilePath to find your profile.