Its not a firmware issue its a power amp issue as was found out. While it may seem that this problem is "common" its actually one of little problems the Mustangs have, so if you search about it you are sure to find a lot of people reporting they have it, making it seem like more common than it really is.
I know a ton of people that have been using Mustangs reguarly for gigging and its been rock solid for them as for me. I have the mustang III but if I search online I find lots of people talking about the fizz issue as well.
Goodluck anyhow !