Nice, but there're some concerns about privacy Rated 4 out of 5 stars
A great add-on to mitigate possible MITM attacks.
However, in its current implementation (version 3.0.3) it compromises user's privacy by potentially letting an adversary know what sites the user visits.
1) DNS request leaks.
The addon performs a DNS lookup when user loads an HTTPS site or (!) switches to a tab where such site is open. Which is worse, it doesn't honor FF proxy settings and performs DNS lookup directly even if a proxy supporting DNS lookups is being used (for instance, user is browsing via Tor).
2) Requests to the Network Notaries.
The addon makes requests to the Notaries to check the certificates. These requests are sent in plain text (as HTTP GET), so any adversary who can see this traffic could know what site was visited by this particular user.