Ah, this is something I've been meaning to address just haven't got around to it. Today I decided I'd look into fixing problems with dark themes and Firefox. Basically, if you're using a dark theme with Ubuntu 8.04 and Firefox 3 a lot of websites will show up with black or dark input boxes and black font inside those. Even some buttons will show up black with black font. This is a huge hassle when it comes to productivity.
To fix the issue, I was able to find a very useful article on Ubuntu's forums. It explains how to edit Firefox's forms.css and the need to install stylish with a few themes. It's pretty straight forward and fixed every dark theme problem that I've had so far.