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
@@ -120,9 +120,6 @@
 % 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
-
 comm.jar:
 % content communicator %content/communicator/
    content/communicator/contentAreaClick.js         (content/contentAreaClick.js)