diff --git a/build-missing-size_t.patch b/build-missing-size_t.patch
new file mode 100644
index 0000000..36c7a1f
--- /dev/null
+++ b/build-missing-size_t.patch
@@ -0,0 +1,11 @@
+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 {
+ 
diff --git a/firefox.spec b/firefox.spec
index ec8c995..cf15343 100644
--- a/firefox.spec
+++ b/firefox.spec
@@ -158,6 +158,7 @@ Patch47:        fedora-shebang-build.patch
 Patch48:        build-arm-wasm.patch
 Patch49:        build-arm-libaom.patch
 Patch50:        Bug-1610814-Fix-NEON-compile-error-with-gcc-and-RGB-.patch
+Patch51:        build-missing-size_t.patch
 
 # Fedora specific patches
 Patch215:        firefox-enable-addons.patch
@@ -364,6 +365,7 @@ This package contains results of tests executed during build.
 %patch48 -p1 -b .build-arm-wasm
 %patch49 -p1 -b .build-arm-libaom
 %patch50 -p1 -b .build-arm-SwizzleNEON
+%patch51 -p1 -b .build-missing-size_t
 
 # Fedora patches
 %patch215 -p1 -b .addons