CSS inserts window won't expand in Firefox 3.6 / Windows XP

I'm using FF 3.6.3 (happened with 3.6.2 also, not sure about other versions of FF) on Windows XP and Atahualpa version 3.4.6.

the CSS inserts window will not expand past one row. It looks fine in Chrome.

I started FF in Safe mode, disabled all WP plugins and it still doesn't work, so it definitely seems like an incompatibility between the theme and Firefox, not related to any WP plugin or FF add on.

Anyone else experiencing this? Very annoying...