diff -up firefox-73.0.1/mfbt/FunctionTypeTraits.h.size_t firefox-73.0.1/mfbt/FunctionTypeTraits.h
--- firefox-73.0.1/mfbt/FunctionTypeTraits.h.size_t	2020-02-26 11:59:48.643893199 +0100
+++ firefox-73.0.1/mfbt/FunctionTypeTraits.h	2020-02-26 11:59:56.806921623 +0100
@@ -10,6 +10,7 @@
 #define mozilla_FunctionTypeTraits_h
 
 #include <tuple>
+#include <stddef.h>
 
 namespace mozilla {