Ok, since i didn't get any answers here, i signed up to the kde forum and asked there. Here's the answer i got, just in case someone else is wondering.
Firefox and Thunderbird are Gtk applications in name only, because they use XUL extensively. As such you will need to locate QtCurve themes for theme seperately, through their respective addon services.
As for Opera, I believe it is similar, only it uses something different. You may wish to try selecting the option "Use system colours" in the theme settings - make sure you are using the shared rather than statically linked version though otherwise it won't work.
I don't think there's any appropriate themes for firefox that will achieve what i want and as for opera, i couldn't get it to take the theme either.