If you run FF3 from a shortcut (desktop, quick launch area, Start>Programs) - which is what I should call 'running FF3') it should be easy: rightclick on the shortcut, choose properties, and set it to run in a maximized window.
When clicking on a link (in, say, an email) it doesn't open maximized, but in a normal window. But you can 'learn' it what window size to use, and that can be "nearly" full screen). To do that, drag and change the window to the desired size and position and close it via the cross in the upper right corner.
Hope this helps.
When running FF3 it always opened up in a full page window. Got a notice for FF 3.1, installed it, now FF3 won't open full size. I have to click the maximize button. Happens every time.