Sorry that you are having a hard time watching your Slingbox from the web browsers.
I had a question for you, what Slingbox model do you have? If you have an older model Slingbox, you can still use the desktop software for that model.
Here is the link if you have a Slingbox model older the the Slingbox 350 and 500:
Awaiting your response!!!
Sling Media Moderation Team
This is on your end Sling. Either on your backend or in your plugin, this is something going on with you.
I reinstalled Windows 8.1 from scratch, only loaded one piece of software: Firefox 29, with only one plugin: your Slingbox player plugin, and it still doesn't work on Firefox. Sits on "Connecting to My Slingbox" until an error pops up saying "SlingPlayer Web Plug-in may be busy, or it may have stopped responding. You can stop the plugin now, or you can continue to see if the plugin will complete."
I can get it to work from Internet Explorer, but not Firefox. Which means at the very least your Firefox plugin is broken.
Slingbox 350, wired directly into Asus RT-N66U router.
OS: Clean install of Windows 8.1U1, no software installed other than Firefox 29 and your plugin. When Firefox 30 came out, updated to that with no change.
Core i7 930 CPU
24 GB PC3-12800 RAM
nVidia GeForce GTX 670 GPU
OCZ-Vertex 4 250GB SSD
WD 1TB HDD
Asus RT-N66U Wireless N Dualband 2.4/5GHz Router.
Comcast 105/25 Mbps Down/Uplink