Why would you want to do something ridiculous like that?? However, if such a need does arise, simply uninstall the extension via the Firefox or Thunderbird extension manager: In the Firefox/Thunderbird file menu, go to Tools --> Add-ons, and find ReminderFox in the list. Then click the Uninstall button. Then delete the reminderfox directory stored in your Firefox or Thunderbird profile directory, such as: C:\Documents and Settings\Administrator\Application Data\Mozilla\Firefox\Profiles\qf5aadhx.default\reminderfox