Fist of all, I have to say that I'm very disappointed with the support offered by this forum. One week after my question and still 0 visits, c'mon!
It would be great that Fiddler autoresponder rules syntax included AND operatos. Just wishful thinking.
So, finally I came up with a script solution. I post it here, just in case someone-sometime-somewhere lands here looking for answers.
static function OnBeforeRequest(oSession: Session) {
//Autoresponder rules
if (oSession["x-ProcessInfo"].Contains("firefox")){
if (oSession.uriContains("example.com")){
oSession["x-replywithfile"] ="C:\\web1.htm";
}
}
if (oSession["x-ProcessInfo"].Contains("chrome")){
if (oSession.uriContains("example.com")){
oSession["x-replywithfile"] ="C:\\web2.htm";
}
}
}