Tough to get help when you provide so very, very little info.
What machine are you using? Which OS version? Which version of Firefox? Which add-on? Have you tried troubleshooting this at all? Have you uninstalled and reinstalled the add-on? Have you tried an alternative add-on that's supposed to do the same thing (if there's more than one, which I believer there are)? Have you uninstalled and reinstalled Firefox? Have you tried an older version of Firefox where you're sure this worked? An older version of OS X where you're sure this worked? Another Mac? A Windows machine?
There are several possibilities for where the issue could lie:
Something could have changed in an OS X update.
Something could've changed in a Firefox update.
Something could've changed in an update to the add-on you use.
YouTube could have changed the way it hides its video files; after all, they don't actually want you to be able to download them, which is why you have to use add-ons to do it in the first place.
Unless you take some steps to narrow it down, there's not much we can really do for you unless we just so happen to have experienced it ourselves.
You also may want to try here, at the Firefox support forums: http://forums.mozillazine.org/viewforum.php?f=38&sid=d3fbd9df711ee5a6b7f15e9a8b588e90
And if your particular add-on has a support website, they may have helpful information for you as well. But in the mean time, the information I mentioned could be helpful in folks here being able to maybe diagnose your problem. You might as well say, "my Honda used to start. It won't anymore. What's wrong?" We need more info.
Thanks for the reply....Can you use any method that you normally use and try it to see if its still functioning...?? I have read other complaints regarding youtube blocking D/L's as of the 14th or 15th citing their user policy..
I don't really download YouTube videos, so I don't have any of those little download add-ons you're talking about. I do use an app called JDownloader for other things though, and it seems more than capable of downloading YouTube videos too at the moment. Just tested it.
However, JDownloader is constantly updated to make finding where the hidden files are easier, which means it could've just already been updated to work around new YouTube barriers. Also, I don't know if it works in at all a similar way to the add-on you're using, so this really doesn't tell us much. Did you try the troubleshooting methods I suggested?