What happens when you try to use Realplayer? Does it freeze, does it give you an error?

Please see also Fix problems with RealPlayer add-ons

Say for instance I open a video on Youtube, it plays but the "download this video" button doesn't appear and "download this video to RealPlayer" doesn't show up on the right-click menu.

Ah, okay. I understand now.

There's a couple of things you could try:

• Try disabling all your your themes, add-ons and plug-ins that are not RealPlayer specific to see if that solves the problem. Do this in Diagnose Firefox issues using Troubleshoot Mode. If doing this gets RealPlayer to work with Firefox, then one of your themes, add-ons or plug-ins (or a combination of two or more of them) are conflicting with the RealPlayer functionality.
  
  
• If you can use RealPlayer in Firefox successfully with a new (unenhanced) profile, or in your standard profile with all themes, add-ons and plug-ins disabled, then try enabling them, one a a time, and restarting the browser after each to test each one sequentially. You should be able to narrow down which one or combination is causing the problem that way.

• If you cannot use RealPlayer in Firefox with a new and unenhanced profile, or with all themes, add-ons and plug-ins disabled, then the issue may be that the RealPlayer product simply does not function as intended. The fact that RealPlayer support has a support page dedicated to the problem seems to indicate the problem is on their end, rather than with Firefox.