Additional: I tried editing the instances of this URL I could find in the advanced config editor in Thunderbird, so I could tell which one was being launched (I added /1 and /2 onto the ends of the URLs) and magically the behaviour stopped.
Additional: I tried editing the instances of this URL I could find in the advanced config editor in Thunderbird, so I could tell which one was being launched (I added /1 and /2 onto the ends of the URLs) and magically the behaviour stopped.