diff --git a/build/mozconfig.cache b/build/mozconfig.cache
index 207bfed578d..c003f5f6ea4 100644
--- a/build/mozconfig.cache
+++ b/build/mozconfig.cache
@@ -60,7 +60,7 @@ else
     ac_add_options "--with-compiler-wrapper=python2.7 $topsrcdir/sccache/sccache.py"
     mk_add_options MOZ_PREFLIGHT_ALL+=build/sccache.mk
     mk_add_options MOZ_POSTFLIGHT_ALL+=build/sccache.mk
-    UPLOAD_EXTRA_FILES="$UPLOAD_EXTRA_FILES sccache.log.gz"
+    mk_add_options "export UPLOAD_EXTRA_FILES+=sccache.log.gz"
     case "$platform" in
     win*)
         # sccache supports a special flag to create depfiles.
diff --git a/client.mk b/client.mk
index 2c156f0128c..bc97d053190 100644
--- a/client.mk
+++ b/client.mk
@@ -195,6 +195,12 @@ $(OBJDIR)/.mozconfig.mk: $(FOUND_MOZCONFIG) $(call mkdir_deps,$(OBJDIR)) $(OBJDI
 include $(OBJDIR)/.mozconfig.mk
 endif
 
+# UPLOAD_EXTRA_FILES is appended to and exported from mozconfig, which makes
+# submakes as well as configure add even more to that, so just unexport it
+# for submakes to pick it from .mozconfig.mk and for configure to pick it
+# from mach environment.
+unexport UPLOAD_EXTRA_FILES
+
 # Print out any options loaded from mozconfig.
 all realbuild clean distclean export libs install realclean::
 ifneq (,$(strip $(MOZCONFIG_OUT_FILTERED)))
diff --git a/configure.in b/configure.in
index a31baae3fbf..390a5ca21f6 100644
--- a/configure.in
+++ b/configure.in
@@ -8895,8 +8895,6 @@ if test -z "$JS_SHARED_LIBRARY"; then
 fi
 AC_SUBST(JS_SHARED_LIBRARY)
 
-AC_SUBST(UPLOAD_EXTRA_FILES)
-
 MOZ_CREATE_CONFIG_STATUS()
 
 # No need to run subconfigures when building with LIBXUL_SDK_DIR