Add conditional check for Fx 67, don't display addon Install button

[devinreams]

No prescribed approach but goal is to not show the install text/button for the addon if not on Firefox 67 or newer.

[jaredhirsch]

125 is related and may be the simpler fix for next week. need to confirm if 66- will be allowed to install anyway if the update manifest includes min version; if so, we probably need this change regardless