I have both firefox and cometbird installed on my system. I just upgraded to CometBird 9.0 and now it won’t start. I’ve had to go back to 8.0 and it works fine. Is this a bug or what?
That depends on why it won’t start. Are you getting any error messages? If not, have you checked your system logs?
I got a message that it couldn’t start and to reinstall it. As for the system logs, I don’t know where to view them.
Uninstall CometBird completely. Go to Program Files and %appdata%/Comet Network and make sure everything is deleted. Run a registry scan with ccleaner(for example) and clean all remaining CometBird entries.
Now install the new version and report back with the results.
SUCCESS!! I followed your steps and that was it. It starts perfectly now, thanks! 
Carpet bombing always works…