diff --git a/pkgs/applications/networking/browsers/firefox/common.nix b/pkgs/applications/networking/browsers/firefox/common.nix
index 1c95b667a4e7..9f0826f0ba08 100644
--- a/pkgs/applications/networking/browsers/firefox/common.nix
+++ b/pkgs/applications/networking/browsers/firefox/common.nix
@@ -292,6 +292,9 @@ buildStdenv.mkDerivation {
     # Runs autoconf through ./mach configure in configurePhase
     configureScript="$(realpath ./mach) configure"
 
+    # Set reproducible build date; https://bugzilla.mozilla.org/show_bug.cgi?id=885777#c21
+    export MOZ_BUILD_DATE=$(head -n1 sourcestamp.txt)
+
     # Set predictable directories for build and state
     export MOZ_OBJDIR=$(pwd)/mozobj
     export MOZBUILD_STATE_PATH=$(pwd)/mozbuild