I think ,the problem lot like firefox tls version ?
From http://fiddler.wikidot.com/mono tell:
If all HTTPS connections to Fiddler fail in Firefox with a "The connection was interrupted" message, you may need to navigate to about:config inside Firefox and edit the security.tls.version.maxpreference. Set it to 1.
mono suppert tls1.0 , security.tls.version.max default value is 3, means firefox suppert tls1.0,126.96.36.199, and when server (fiddlercore)not suppert tls1.2 , can fallback to tls.10 ,why need set security.tls.version.max to 1 ?