Add explicit Firefox entry to browser list

I look the current [browser list](http://browserl.ist/?q=Chrome+>%3D+45%2C+Firefox+ESR%2C+Edge+>%3D+12%2C+Explorer+>%3D+10%2C+iOS+>%3D+9%2C+Safari+>%3D+9%2C+Android+>%3D+4.4%2C+Opera+>%3D+30)

And I have one question: why do you support only one version of the firefox browser?

![image](https://user-images.githubusercontent.com/4661784/33378131-ed104c6e-d524-11e7-8fc2-169299f94b27.png)
