Download Firefox 51.0.1 then in the URL box type "about:support" then look for the line that says "Multiprocess Windows - 1/1 (Enabled by User)" that indicates whether electrolysis is enabled. If it says "0/1 disabled" you will have to manually enable it.
Next you want to check to make sure your WebGL, WebGL2 and GPU rendering is accelerated under Graphics. If that's all good and your GPU is listed, go scroll up to the top right and click "Refresh Firefox"
when you restart it, go back into "about:support" and make sure everything is loading fine. Then you can head over to test it out http://beta.unity3d.com/jonas/WebGLBenchmark/ and then you should behold the power! Firefox should benchmark and run websites very noticeably faster than other browsers. The only exception is Firefox's inability to render forum list heavy sites (It chokes on them), so all other browsers will out perform it on Facebook, Flickr and 500px for the time being until that gets fixed. In fact if you are a heavy Facebook user, Safari performs the best with it, and if your on Windows then Opera gives the best Facebook performance. You can also type in "about:performance" to get a rundown if any plugins are fucking you around.