I just recently had my Macbook's HD replaced, and some new problems have arisen since I got it back. First, if I try to play any in-browser media (like streaming radio, or video), Safari just mysteriously exits and it gives me the option to report the unexpected error that caused it. Also, I have tried multiple times to install firefox, but whenever I click on the downloaded icon (the one that looks like a white hard drive with the firefox logo), all the icons on the desktop just vanish for a second, and no firefox opens. It's getting extremely frustrating. Does anyone have any solutions?

Thanks