There's a review by Sk8er that mentions a problem with CyberGhost not being able to be used with HTTPS only mode enabled in Firefox.

Sk8er is correct - this is an issue - although it's not the issue that I experienced (I'm using an older version of Firefox on Linux, that doesn't have the HTTPS Only feature).

The issue that I experienced also involved the "on/off" switch not being able to be switched on, but seems to relate to the FoxyProxy addon.

After disabling the FoxyProxy addon, it seemed to fix the issue. After re-enabling FoxyProxy, the CyberGhost addon now works fine even with FoxyProxy enabled.