See: http://real.custhelp.com/app/answers/detail/a_id/7303/p/3/c/2222/r_id/119487

Real is always, it seems, late in releasing updates for new browser updates. They are given the same opportunity during the Firefox pre-release testing as other developers to be sure that their products will work with new Firefox releases.

But, alas, always the same problem!

it is the add ons problem not the firefox or real player because it works on ie so check your add -ons if you install adb block plus it some times block . disable adb and check again i hope it will solve

I first tried firefox 3.5.15, it did not work,

but later, there was an automatic updated happened,
and I found out that all of sudden, by chance, that it is working,
the button download start to work,  so I checked out the update, 

it is the 3.5.16 (20101130074636) update.

details please see the image.  Hope this will work for you.  

I first downloaded 3.5.15 from oldapps.com   

then the auto update, and once it start working, i stopped the further update from:

tools - option - advanced - update - choose the manual update option, disable the automatic update. 

hope this will help you.

Thank u for the tip,this fix worked for me also!Maybe I should be clearer on this the Firefox version 3.5.15 works but not the updated version 3.6.13