Mozilla isn't going to fix any problems with Skype, that is up to Skype to do.

Exactly what is the problem with Skype as it relates to Firefox?

I wouldn't recommend going back. I mean, some older versions like 3.6 may be more stable in certain areas, including Skype, but that is only being supported for a short time. Once a version looses support you become more prone to security threats, bugs and other such consequences. At least with the newer versions there is not only a constant support network but also new updates every day, so if you can deal with the small bugs for a while until they get fixed you'll be in good shape.

Read the 2nd post in this thread about Skype: http://community.skype.com/t5/Windows/Recently-upgraded-to-Firefox-5-and-Skype-extension-not-working/m-p/1348/highlight/true#M212