A preference is any value or defined behavior that can be set (presumably, one setting is preferable to another). Preference changes via user interface usually take effect immediately. The values are saved to the user profile (in prefs.js), for both Firefox and Thunderbird.

A preference is read from a file, and can call up to four methods: pref(), user_pref(), sticky_pref() and lockPref(). All preferences files may call pref(), user_pref() and sticky_pref(), while the config file in addition may call lockPref().