They're kept in a file at /home/USER/.mozilla/firefox/XXXXXXX.default/cookies.sqlite - where USER is your username and XXXXXXX is the default profile name.
As you may have noticed, they're kept in a single sqlite file. You won't be able to just open one cookie and edit it using a text editor or anything like that. I'd recommend using a Firefox extension for that purpose. There are a number of them available.
AMD Phenom II X6 1045T 2.8GHz 8GB DDR3 RAM ATI Radeon HD6970