A mere three days over a year after the first person asked, here’s a Mozilla and Firebird extension to add a Blogger “BlogThis” item to your right-click context menu. To quote the Blogger API page, its status is experimental and alpha (translation: it works for me, I’m posting to find out if it works for you, so please let me know).
Still to come: an MT It! version for Movable Type (which will require that I learn how to do extension settings) or possibly a generic version where you just give it a URL with placeholders for the URL, title, and selected text. In a while: settings are much harder, plus they require that you have some design sense (or steal from people who do).
Thanks to Ted Mielczarek, from whom I stole most of the code: I finally learned that there is no documentation, so the only way to learn how to do extensions is to find one that does something similar (or find a similar bit of functionality in the browser itself, though that’s harder), and shamelessly steal from it.