A prefbar button that opens the standalone cookie manager (chrome://communicator/content/permissions/cookieViewer.xul), which I first posted on alt.comp.software.seamonkey on USENET. As stated in that post (links adapted to HTML),
This is basically based on prefbarOpenPrefs() from PrefBar (prefbarOverlay.js) and I adjusted the openDialog() call to be like the one in bug 476274 (see commit).
Note that if opening this cookie manager inside a browser tab is OK for you, then adding a bookmark, as suggested by frg in the same thread, works too.
I did this mostly to try to play with creating custom Prefbar buttons, and I'm sharing the result in case it's helpful to anyone.