--- a/configure.in
+++ b/configure.in
@@ -3905,17 +3905,17 @@ else
   # "mobile" no longer exists.
   if test "$MOZ_BUILD_APP" = "mobile" ; then
     AC_MSG_RESULT([none])
     AC_MSG_ERROR([--enable-application=mobile is no longer supported.])
   fi
   # Support comm-central.
   if test -n "$EXTERNAL_SOURCE_DIR" ; then
     MOZ_BUILD_APP="$EXTERNAL_SOURCE_DIR/$MOZ_BUILD_APP"
-    MOZ_BUILD_APP=`${PYTHON} -c "import os.path; print(os.path.relpath(\"${MOZ_BUILD_APP}\", \"${srcdir}\"))"`
+    MOZ_BUILD_APP=`${PYTHON} -c "import mozpack.path as mozpath; print(mozpath.relpath(\"${MOZ_BUILD_APP}\", \"${srcdir}\"))"`
   fi
   # We have a valid application only if it has a build.mk file in its top
   # directory.
   if test ! -f "${srcdir}/${MOZ_BUILD_APP}/build.mk" ; then
     AC_MSG_RESULT([none])
     AC_MSG_ERROR([--enable-application value not recognized (${MOZ_BUILD_APP}/build.mk does not exist).])
   else
     AC_MSG_RESULT([$MOZ_BUILD_APP])