Add new Firefox versions

[ExE-Boss]

Mozilla have released Firefox 59 and will soon be releasing Firefox 60.

Also, the Firefox (all flags enabled) option is outdated and scores lower than Firefox 58.

[scheinercc]

This has been partly addressed in #530. Maybe this task can now focus on "all flags enabled" only?

[dstorey]

Flags in Firefox are difficult as there are flags for every little thing and it is hard to find the relevant ones (reminds me of when I was at Opera :D) I'll try to see if I can find anything relevant

[scheinercc]

Thanks @dstorey