diff --git a/aclocal.m4 b/aclocal.m4
index d71c4a00476a..76ab83f8e69e 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -13,7 +13,10 @@ builtin(include, build/autoconf/libart.m4)dnl
 builtin(include, build/autoconf/pkg.m4)dnl
 builtin(include, build/autoconf/freetype2.m4)dnl
 builtin(include, build/autoconf/codeset.m4)dnl
-dnl
-define(MOZ_TOPSRCDIR,.)dnl MOZ_TOPSRCDIR is used in altoptions.m4
 builtin(include, build/autoconf/altoptions.m4)dnl
 
+# Read the user's .mozconfig script.  We can't do this in
+# configure.in: autoconf puts the argument parsing code above anything
+# expanded from configure.in, and we need to get the configure options
+# from .mozconfig in place before that argument parsing code.
+MOZ_READ_MOZCONFIG(.)
diff --git a/build/autoconf/altoptions.m4 b/build/autoconf/altoptions.m4
index 756f5df5f586..f9db0a539e25 100644
--- a/build/autoconf/altoptions.m4
+++ b/build/autoconf/altoptions.m4
@@ -152,6 +152,3 @@ AC_DEFUN([MOZ_READ_MOZCONFIG],
 # See the mozconfig2configure script for more details.
 _AUTOCONF_TOOLS_DIR=`dirname [$]0`/[$1]/build/autoconf
 . $_AUTOCONF_TOOLS_DIR/mozconfig2configure])
-
-dnl This gets inserted at the top of the configure script
-MOZ_READ_MOZCONFIG(MOZ_TOPSRCDIR)