Which version of Firefox are you using? I have found that the most recent version (Firefox 7.0.1) does not (yet) work with the Slingbox. I have downgraded Firefox on all computers that I use to connect to my Slingbox to Firefox version 6.0.2, and full functionality has been restored.
CORRECTIOIN EDIT BY CHAKKARINEN
Many thanks to phrelin, who found the solution to the Firefox 7 update issue. He writes:
"After I installed FF7 in its own directory and clicked on my Sling bookmark and the "watch" link, it went through the updated add-on install routine. It works fine."