Some of my supervisor’s fourth-year students are working with Thunderbird for their final-year projects. He asked me to give them a brief primer on how to write extensions. My initial reaction was "go search the Web", but I put in the work and ended up saying "go and search the Web on these sites". Since I threw together some links to useful resources, as well as a selection of what might generously be called "tips", I figured I’d link to it in case it’s of use to anyone else. So, Thunderbird Extension Tutorial(-ish).