[TYPO3-english] rtehtmlarea backend rendering problem

[bw at shs-it.de]

Hi there,

I have a custom extension with an rtehtmlarea in the backend, but I have 
a rendering problem.
Here's a screenshot:

http://www.doubleu-it.com/rtehtmlarea%20backend.jpg

I'm using Firefox 3.6 under Win7.
Strange: Firefox 3.6 under Max OS X doesn't have this problem!

I have no idea how I could fix this - anybody does's?
Thank you very much!
Greetings,
Ben