Since this is a known limitation of the current implementation, you should move this post into the Enchancement Request forums. Just use the control panel on the right-side of this page. It'll probably carry more weight than trying to get someone to troubleshoot your issue in the technical support forums.
I do know however, that Sling Media has annouced that they're working on a flash version of their Slingplayer. Which would make the application more universal. If you're interested, there's also an article, (http://placeshiftingenthusiasts.com/watching-your-slingplayer-on-linux/), that describes how to get Slingplayer to work within WINE.
For now, screw Firefox/Flash/Ubuntu... I have a Linux solution! I stumbled upon this completely by accident and haven't tested it yet because some compiling and patching is involved. If you would like to know the solution, please PM me or message me as I do not think this should be public just yet. For one reason, it's quite beta. Secondly, it's not officially supported by SlingMedia, who seem to not care if a Linux version is ever created.