I've seen this before with Firefox. Basically Firefox has disabled weaker certificates from working, where Chrome and IE still accept ones with 128bit encryption, they do show an error (at least in Chrome) if you dig into the SSL debug screen. Firefox just refuses to view it.
Ah, I have read somewhere on the internet that AES-256 would be less secure than AES-128 because the AES algorithm has been designed for 128bit.