Boy, I've gone around and around on this one, so please forgive me if this is still not the appropriate place for this. Feel free to point me in the right direction, or straight to hell, if that is more suitable...but here's the thing...
I use Thunderbird to read RSS feeds from many sites, most notably theCHIVE, and when I read a new message from their feed (for example http://feedproxy.google.com/~r/feedburner/ZdSV/~3/3DMhCjetspI/), it forces open a new browser window (in my default browser, Firefox, not in Thunderbird itself) and loads https://twitter.com/i/jot, a 1x1 pixel gif image.
This has also happened on a few other sites, but it is most reliably reproducible on theCHIVE (http://www.thechive.com/) and its sister site theBERRY (http://theberry.com/) from their RSS feed when read in Thunderbird.
I have finally traced it to a line of code in the Twitter share button that calls http://platform.twitter.com/widgets.js
Here's the line: