Webgl build alerts to download Firefox in Firefore

I get the following error in Firefox 43.0.1. in WebGL built from Unity 5.3.1f1.

“You need a browser which supports WebGL to run this content. Try installing Firefox.”

This usually means that it can’t start a WebGL context or the browser for some reason isn’t supporting WebGL.
What does this http://webglreport.com/ say about your browser?