AGAIN?! -- I groaned. Yes, it was not the first time Firefox froze because of the plugins.
I did what I usually do -- Google. And finally I understood it was a special feature Adobe had added for Firefox, the Protected Mode, that caused the problem. The Protected Mode is supposed to be a new security enhancement for Firefox only. It can be turned off if it causes problem to your browser. BUT, how many users would bother to do the research and learn to turn it off? They would only get frustrated and turn to IE or Google Chrome. Your friend can become your worst enemy.
To turn off the Protected Mode of Flash Player, you can follow this link. If you are too lazy to click, you can basically do this:
Edit a file named mms.cfg in C:\windows\system32\macromed\flash and C:\windows\syswow64\macromed\flash (64bit). Add one line into it:
If the file does not exist, create one.
This problem is only on Windows system so far.