Problem while opening the latest version of Avant

Hi all,
I just got the most up to date version of Avant and installed it. A message came up telling me to do so.
When I try and open it or use the URL, a white screen and the following message appear:

XULRunner
Error: Platform version '11.3' is not compatible with
min Version > = 12.0
min Version > = 12.0
Do you know what I can do to solve it?
Lots of thanks.
