HTML too large for message compose window

With HTML Editor on for messaging, the message compose window is too small to hold the HTML Area editor and the Send Message button,

The HTMLArea widget is too wide, and overflows to the right. The send button appears too low on the frame, and requires scrolling down to press.

Clearly visible on 1.8.x and HEAD (1.9). On Moodle v1.6 the sizes were correct – and quite pleasant to use.

This is on Firefox 2.0.0.6 Windows.