I was able to install that extension in Firefox 40.0.2, but the Toolbar Button doesn't appear and it isn't in the Customize > Additional Tools and Features palette. My guess is that it uses an older API that was removed for Firefox 40.

So is it going to be fixed?

Yes, this extension uses the SDK Widget module and support for this module has been removed in Firefox 40.

• let sdkWidget = require('sdk/widget').Widget(options);

The developer of the extension needs to fix this in an update and use the 'ui' module instead. The Widget API has been marked as deprecated since Firefox 29 and now been removed in the current release, so developers had enough time to fix this and ensure that their extension keeps working properly.

• https://blog.mozilla.org/addons/2015/06/18/compatibility-for-firefox-40/

• https://developer.mozilla.org/en-US/Add-ons/SDK/High-Level_APIs/widget
• https://developer.mozilla.org/en-US/Add-ons/SDK/High-Level_APIs/ui

I won't be updating Firefox until it is fixed

Appears to work now

An updated version of - https://addons.mozilla.org/en-US/firefox/addon/rt-news/ - was released and is working now in Firefox 40.0.3 .