I'm not sure how I missed this before, but there is FINALLY a Firefox extension for my beloved OneNote! It was originally written by Gmx Lee.
Patrick Schmid mentions that it was designed for OneNote 2003 and FireFox 1.5 and provides a version for FireFox 2.x and OneNote 2007.
Patrick was a great contributor during the Microsoft Office 2007 beta program and maintains a super site for all things 2007-ish.
via Patrick Schmid