Replace Firefox in place of Firefox nightly icon

[ranjithrajv]

Firefox 55 which is released on August 08, 2017 adds WebVR support.

So, I think we can replace Firefox in place of Nightly now. Check https://webvr.rocks/ for verification.

Tags: Help Wanted, easy

[ranjithrajv]

@ngokevin: Added a pull request, please review.

3f59439