Forgot about the clipboard
I'll have to look it up to see if it's been changed, but last time I checked, FireFox doesn't let a Web page interact with the clipboard. It's one of several extremely paranoid restrictions in FireFox, though to be fair, it can be used to invade privacy. When I built a "honey trap" site to play a Nigerian scammer, I set it up to read his clipboard. One time I was able to capture a long list of email addresses of his next victims that he'd just pasted into an email, and had a little fun sending them all warnings.
With great power comes great responsibility...