A few weeks ago, I upgraded to Windows 7 Home Premium, 64-bit. I was using only IE9 until yesterday. I installed Firefox 4.0.1. Out of curiosity when I saw AZ's post, I tried DISH Online in FF 4.0.1. I followed the prompts to install the Flash plug-in, then the Sling plug-in. Live TV works for me.
What about content that isn't LiveTV, isn't from your DVR, and isn't from Hulu...
Dish Online's main experience isn't from LiveTV/DVR ... much of the "DishOnline" experience is either Hulu based, or premium "subscriptionized" content, Epix, HBO, etc.. its the content that comes from those sites that has the problem with Widevine Media Transformer (or DRM by another name) and Firefox 4..
For me.. I've never had problems with FF4 and DishOnline doing LiveTV, or DVR'd items.. and initially I didn't have problems watching other content.. but then someone at D.O. turned on this supposed "optimizer" and ever since I've been unable to get any of that other content (going on 2 months now) and also with no response from "feedback" ...
EDIT: "haven't had problems" ... with the exception in other threads that two sundays ago, "THEY" did something at DishOnline and since ... any time I try to play a DVR event through DO's site.. it starts playing on TV1 rather than starting *only* on the web interface ... and with no button to select "play on web" or "play on tv" it kind of kills the use of your Sling Adapter if it starts playing it on TV1 pissing off the people in front of that television... but this problem happens FF or IE