I have a suggestion, why not make the textarea for the message/reply larger? All you've got set is the cols attribute, which no browser seems to render accurately but it's particularly small in the latest version of Mozilla (only about 30 or so characters wide). Why not add in:
So that the textarea actually fills the space available in the table and thereby is rendered the same size in all browsers (that support CSS, anyway)? You could do the same for the subject input as well. Also, on a side topic, if you've disabled the use of fonts, why not get rid of the font menu?
<textarea name="message" rows="20" cols="40" wrap="virtual" tabindex="2">
<textarea name="message" rows="20" cols="40" wrap="virtual" tabindex="2" style="width:100%;">
I think I may have been here before... a long, long time ago... 599699 Unread Posts, you say?
Ah, I found my old avatar. The hypnotic evil duck.
It was 2002. Duck avatars were the style at the time.
Long and fascinating story there. I'd tell you if I remembered it.