diff --git a/testing/firefox/build b/testing/firefox/build
index 6a2283bd..11ab6103 100755
--- a/testing/firefox/build
+++ b/testing/firefox/build
@@ -19,6 +19,10 @@ cd firefox
 
 mv mozconfig .mozconfig
 
+# Fix OOM errors.
+cores=$(nproc)
+printf '%s\n' "mk_add_options MOZ_MAKE_FLAGS=\"-j$cores -l$cores\"" >> .mozconfig
+
 for patch in *.patch; do
     patch -p1 < "$patch"
 done
diff --git a/testing/firefox/files/mozconfig b/testing/firefox/files/mozconfig
index cc3b39c6..fb664191 100644
--- a/testing/firefox/files/mozconfig
+++ b/testing/firefox/files/mozconfig
@@ -38,6 +38,3 @@ ac_add_options --enable-alsa
 ac_add_options --disable-dbus
 ac_add_options --disable-necko-wifi
 ac_add_options --disable-startup-notification
-
-# Prevent OOM
-mk_add_options MOZ_MAKE_FLAGS="-j7 -l7"