I adapted my code from BlueGriffon to Firefox this morning. Apparently Tim Taubert independently had the same idea - and also the same UI - a while ago. I promise I did not know about his code when I started mine. Anyway, you can find my XPI for Firefox 5+ here. Warning, in the case of Firefox, it's still highly experimental, can lead to severe issues because of bug 707633. Restart Firefox after installation and you'll find a new panel in the Preferences window.

(if you experience long delay between the click on the XPI link above and the "Install add-on" dialog, that's bug 657581)