I'd try just reinstalling it before I did anything else.
It may just have gotten something messed up during the upgrade or install.
I'd remove the original install and install it in a new location.
That helps stop it from picking up any errors from the previous install.
If it's in a different location Windows pretty much sees it as something new.
For instance you could just install it to C:\Firefox, you could even put it on another drive if you want to.
If you don't have Revo Uninstaller, you could download and run it.
It uninstalls your software and get rid of the junk that Windows leaves behind.