I waited with this remark till I upgraded to the latest zimbra version.
That now has been done. It went without a problem, thank you very much (debian4 zimbra 5.012 -> debian 5 zimbra 6.08p2)

But the problem - ok minor annoyance - is still present:

if, while replying to a mail in the AJAX webclient, I switch to another program (fe. explorer to search the whereabouts of an attachment) and afterwards come back to the editing window, I can no longer enter text.
I need to save the mail as draft and reopen it in order to enter text again.

Scratch above remark, I just discovered it seems to be a firefox problem.
I got the same behaviour while editing this post. however it only occurs when switching to an application other than firefox.
The functionality can be restored by temporarily switching to another ff tabpage

So, not a zimbra problem. Sorry