Make sure global hotkey page is compatible with VTR

[Croydon]

Coming from #237

[Croydon]

This page got introduced in Firefox 66.

Therefore I will ensure compatibility in the first VTR versions which increases the minimum Firefox version from 60 to 68, which is likely 0.15.0

https://www.mozilla.org/en-US/firefox/66.0/releasenotes/