Remove about:buildconfig.  If used as-is, it would add unnecessary runtime dependencies.
--- a/comm/mail/base/jar.mn
+++ b/comm/mail/base/jar.mn
@@ -132,8 +132,6 @@
 % override chrome://mozapps/content/profile/profileDowngrade.js chrome://messenger/content/profileDowngrade.js
 % override chrome://mozapps/content/profile/profileDowngrade.xhtml chrome://messenger/content/profileDowngrade.xhtml
 
-*   content/messenger/buildconfig.html              (content/buildconfig.html)
-% override chrome://global/content/buildconfig.html chrome://messenger/content/buildconfig.html
 % override chrome://global/locale/appstrings.properties chrome://messenger/locale/appstrings.properties
 
 comm.jar: