Add Firefox and Webkit

[pietdevries94]

In nixpkgs, @phaer worked hard to add support for Firefox and Webkit.

NixOS/nixpkgs#298944

I personally didn't need this, so I hadn't looked into it, but I think it's a good idea to port this. I don't think I have the time in the coming weeks, but will do so. This issue is more a promise to the user base of this flake than anything else.