I cannot install Flash.

So, yesterday, Flash stopped working in Firefox. I recently updated Firefox to 18.0.1. What would happen is I would go to a website like Youtube and try to play a video, and it wouldn't even load it. The program would freeze, and then say, Flash Player has crashed. So, I uninstalled Flash, but now I cannot re-install it. It will let me download the program, but whenever I try to open it, my computer just sits there. Often times, when I open Task Manager, it says the program is running in the background, but I've waited up to 15 minutes for the program to prompt me to continue and nothing. I am running Windows 8, and I have Norton Internet Security for my anti-virus software. I do not know what's causing the program to fail to run, but any help would be much obliged.