Yes, it is Firefox 3, on Windows 2000.
Indeed, it seems to increase the size of the text box with text size, but not to a big enough size.
I turned off Firefox "minimum font size" (advanced) but that doesn't help.
My default font is set to Lucida Sans Unicode, size 14. Changing it to 12 doesn't help (in fact you seem to be overriding all the default fonts).
Um ... what can I say... it's nearly a bog standard ff3 install ... I disabled all add-ons...