If memory serves me Firefox cookies are stored unencrypted in an sqlite database with user level permissions. Haven't dug too deep, so I could be missing something, but last time I peaked down the rabbit hole that's what I remember.
http://raidersec.blogspot.com/2013/06/how-browsers-store-you...
$ find "$HOME/Library/Application Support/Firefox/Profiles" -name cookies.sqlite -exec sh -c "sqlite3 '{}' 'SELECT * FROM moz_cookies'" \;They are encrypted at rest, and encrypted before sync.