Error opening new version of Avant

Hi
I am an old user of avant. I just receives a message today to install the new version which has just been released.. I installed the newest version of Avant. It was installed successfully but now when I am trying to open it, I end up in a blank screen and an error window pops up. The error message is attached below:
"XULRunner
Error: Platform version '11.0' is not compatible with
minVersion>=12.0
maxVersion>=12.0"
Can anyone explain why this error is coming? I have no idea.
If anyone has used avant please do let me know. I'll be thankful.
Regards
Edison
