I have two computer that I watch my slingbox 500 through. They both use firefox and just recently one of them no longer brings up the remote when I click on the remote icon. I have tried removing the plug in and reinstalling it but that did not help.
All you need to do is copy the remote image files from the working computer to the non-working one. They are stored at C:\Users\..YourName..\AppData\Roaming\Sling Media\WebSlingPlayer\Remote\ You will need to check "Show Hidden files" to see the \AppData\ folder.