If you updated a newer version of Firefox, you probable have noticed that the icon of "Download this video" on the up right of Youtube videos disappeared. You no longer be able to download youtube videos using RealPlayer. This happened several times once Firfox newer versions came out.
Here is solution I found from Internet search and it does work (credit goes to Hest):
step 1: type about:config in the address bar
step 2: in the filter box type dom.ipc.plugins.enabled.npswf32.dll.
Step 3: change the value to false.
step 4: restart FireFox