Problems with my iBook G4/Firefox!?

I was on a website and I got a link sent to me, being a curious idiot I clicked on it and it took me to a site with a rather disturbing video. I then tried to click on the video but the window kept moving around. I finally outsmarted the window by placing my arrow in the general are of where it kep jumping and clicked 'X' on the tab. Then a box popped up, and it kept coming and coming everytime I hit enter. I got really frustrated and held down my power button to shut my computer off. I then turned it back on, clicked Firefox and my entire screen went grey and a thing came up that told me to "hold down my power button so I can restart the computer" I did this, and then clicked Firefox again once I did, the same thing happened. I shut the computer off, turned it back on and used Safari.

I want to know what I can do to make it stop doing this? I need a lot of help, I know it's a lot to ask - but really you'd be saving me so much grief.

Thanks in advanced.

Chelly.