Hi ssquirelss, Signed Add-ons are new in version 41 of Firefox. Back in February it was announced that for a safer Add-on experience all extentions would have to be verified and signed by Mozilla before they can be installed.

This happens automatically for extensions hosted on addons.mozilla.org. If you are using Add-ons distributed by third-parties, they have to submit extensions to be signed first. Please inquire at the vendor of such an Add-on if they have an updated and signed version available.

The signing requirement will become mandatory in regular release and beta versions of Firefox after Firefox 42. For testing purposes it will still be possible to configure Firefox Developer Edition and Nightly versions to run unsigned addons though, for details on this see https://wiki.mozilla.org/Addons/Extension_Signing.

guigs the Unity Web Player is a Plugin so it is not affected by the new Extensions signing requirement.

ssquirelss, the reason for the Unity Web Player plugin not working on the Nightly builds you are Testing is because it is Win64 and not 32-bit on Windows.

As of Firefox 41.0+ the Win64 builds regardless of channel it is on currently at time (whether Release, Beta, Aurora or Nightly) only has the 64-bit Flash Player plugin white listed. All other 64-bit Plugins will not work on Win64 Firefox 41.0+ currently.

If you want to use Plugins on Windows from Firefox 41.0+ builds you need to use 32-bit Plugins and 32-bit builds.

The 64-bit Firefox builds for Mac OSX and Linux will still be able to use any 64-bit Plugins that exist for the OS's besides Flash Player.

Firefox 41.0 Release is going to be the first release to have Win64 (64-bit) alongside the 32-bit on Windows, unless it changes at last minute.