Firefox 40 is set to “WARN only” allowing some time for 3rd party vendors to come up to speed.  Still, this is a beneficial change based on past user experiences where malware infections can more easily occur without this needed control  

Firefox Brings Fresh new Look to Windows 10 and Makes Add-ons Safer
http://www.cnet.com/uk/news/firefox-tries-to-edge-out-microsoft-in-windows-10-browser-battle/
https://blog.mozilla.org/blog/2015/08/11/firefox-brings-fresh-new-look-to-windows-10-and-makes-add-ons-safer/

Released on Tuesday, Firefox 40 is the latest version of Mozilla’s browser and the first designed with Windows 10 in mind yet with its own look and feel. The icons sport greater contrast to better stand out. The screen opens up more real estate for the actual web pages. And a minor renovation paints the top and bottom areas of the browser a more user-friendly gray instead of the standard white. There’s one more change in the new version of Firefox aimed toward beefing up security. Browser add-ons can sometimes be a source of malware, most notably those that aren’t certified. Starting with Firefox 40, Mozilla will now certify all add-ons that you attempt to install. For now, the browser will simply warn you that the add-on is uncertified, or unsigned. But in future releases, Firefox will actually disable any add-ons that are not signed.