Earlier quoted context omitted.
> Its possible to MITM an HTTPS connection Not in the case of Firefox connecting to AMO, because it uses a pinned certificate for that.
Are you sure? I perform HTTPS interception on all out-bound traffic on my network and I don't recall making an exception for AMO, and I have a number of add-ons installed. Though, it wouldn't be the first time I've forgotten about something like that.
Did you install your addons before updating to Firefox 32? That's when the pinning was introduced.