Firefox cookie database
Cookies in Firefox are stored in an SQLite format database found in the file cookies.sqlite in the currently-active user profile directory (exact path is system-dependent). Also, the write-ahead-logging and shared-memory files cookies.sqlite-wal and cookies.sqlite-shm are used, but the latter two are re-integrated into the main database file and deleted when you close the browser.
The structure is seen in this SQL command embedded in the file:
CREATE TABLE moz_cookies (id INTEGER PRIMARY KEY, name TEXT, value TEXT, host TEXT, path TEXT,expiry INTEGER, lastAccessed INTEGER, isSecure INTEGER, isHttpOnly INTEGER, baseDomain TEXT, creationTime INTEGER)