"." is a regexp special char. We mean "." literally so it ought to be escaped.

Wrong:

 $ echo 'user_pref("extensionsXenigmailYconfiguredVersion",' \
   | sed --regexp-extended \
        '/^(user_)?pref\("extensions.enigmail.configuredVersion",/d'

Right:

 $ echo 'user_pref("extensionsXenigmailYconfiguredVersion",' \
   | sed --regexp-extended \
        '/^(user_)?pref\("extensions\.enigmail\.configuredVersion",/d'
 user_pref("extensionsXenigmailYconfiguredVersion",