For some strange reason the flash folder in C:\WINDOWS\system32\Macromed was locked by explorer.exe. Once the Adobe Flash Player is installed in this folder, the locking prevented it from being copied into both Firefox’s plug in folder in C:\Program Files\Mozilla Firefox\plugins and Iexplore’s plugin folder in C:\Program Files\Internet Explorer\PLUGINS, such that from the plugin point of view of Firefox or Iexplore, the Adobe Flash was not installed. So once you browse through a flash content, like Youtube, it constantly asks you to install Flash player. However, reinstall flash or firefox will never have this issue solved. Somebody blame RealPlayer for this. But disable/uninstall Realplayer, disable all other plugins, will not change the locking of this folder. Problem remains.

Workaround:
1. In taskmgr.exe, kill explorer.exe. Explorer will close (with your desktop) and restart itself and bring your desktop back. This will release the lock.
2. Do not do anything but using my computer to navigate to C:\WINDOWS\system32, delete the folder Macromed.
3. Using either Firefox or IE to navigate to www.adobe.com, download and the newest version of Flash Player. Do not run the installed, but open the download folder.
4. Close both Firefox and IE, install Adobe Flash by running the installer in the download folder.

Now Adobe Flash will be running again in both Firefox and IE.

Enjoy.