Firefox crashes upon startup after update to new version and download attempt

I was asked to update to a new version of Firefox, and when I responded positively to something like, "Do you want to run Firefox now?" it crashed immediately. When I tried to launch it from the desktop it crashed immediately. When I launched it holding down the shift key it brought up Internet Explorer, not Firefox. I don't know why. I tried to download Firefox from scratch, but the result was the same--it crashed upon launch.