Right now, plugins don't have much access to the mozilla core, in
fact, the only things they can really do to mozilla is access prefs,
go to a url, and access the history - all the things that that the
current plugins need. The mozilla access is flexible so that when
Brian or I need to make a new plugin we say "hey, I need to get to
this piece of mozilla", and then we code the function into kmeleon,
and then write a plugin to call it.
Making a cookie managment plugin would probably be pretty similar,
you'll need to say "I want to be able to add, remove, and list the
cookies", so you'll add those functions to Plugins.cpp, and then call
them from your plugin.
You'll need to do some research to figure out how mozilla handles
cookies, and what elements it exposes to embedders.