Silverlight not detected by Firefox Developer Edition and does not appear in plugins
Silverlight is installed and works with other browsers, but does not show up in plugins in the addons menu, and no silverlight sites will work.
Wšě wotmołwy (2)
Currently 64-bit Developer Edition only supports Flash plugin in the 64-bit flavor. Support for other 64-bit plugins will be added in the future.
If you need Silverlight you'll need to install the 32-bit version of Developer Edition.
The Win64 Firefox builds as of 41.0 only has 64-bit Flash plugin white listed
Since you are Testing the Aurora aka Developer Edition channel builds you have seen this change sooner due to a recent checkin.
This does not effect the 64-bit builds on Mac OSX and Linux.
Wot James