If I use Internet Explorer (I keep it on the machine, but don't normally allow it access through the firewall), PPRuNe works normally.
You've found the solution to your problem yourself. I've never found a reason to stray far from IE and OE. I'm on Ver 6.0 SP1 and find they are solid.
Whichever Browser you are going to use, keep it up to date with the patches available from their Home Sites.