HTTPS Proxy
Supported on: Firefox 60 or later, Firefox 60 ESR or later
Registry
HKLM
Software\Policies\Mozilla\Firefox\Proxy HKCU
Software\Policies\Mozilla\Firefox\Proxy Options
SSLProxy text Description
If this policy is enabled, you can set the HTTPS Proxy used when manual proxy configuration is specified. If this policy is disabled or not configured, Firefox does not use an HTTPS Proxy.