Firefox crashing problem! PLEASE HELP!!?

When ever I try downloading something, or anything that involves youtube or google images, I get an error telling me firefox is not responding. I exit firefox and I reopen it and it says it's already running but and I need to close it. What is up with this? I have the newest firefox update so I shouldn't have any problems. I've cleared all my cookies and cashe but it's still crashing. Also for some strange reason my homepage keeps changing on me.