I'm running FF2 on Linux and I'm running the latest RoundCube version 0.1.1.
The preview pane is not auto adjusting. The message listings are set at a static size. I can provide screen shots if it will help, or whatever else you require to figure this out.
Your screen shots on this discussion page display a spell checker in the composition page, but I don't have one anywhere. Can you tell me how you set that up?
Again, I will do whatever I can do to help. Just ask.