fix: don't globally enable firefox env

users/common/programs/firefox.nix

@@ -1,16 +1,31 @@
-{
-  home = {
-    sessionVariables = {
-      # Firefox touch support
-      MOZ_USE_XINPUT2 = 1;
-      # Firefox Hardware render
+{ lib, pkgs, ... }:
+let
+  inherit (lib) concatStringsSep escapeShellArg mapAttrsToList;
+  env = {
     MOZ_WEBRENDER = 1;
+    # For a better scrolling implementation and touch support.
+    # Be sure to also disable "Use smooth scrolling" in about:preferences
+    MOZ_USE_XINPUT2 = 1;
+    # Required for hardware video decoding.
+    # See https://github.com/elFarto/nvidia-vaapi-driver?tab=readme-ov-file#firefox
+    MOZ_DISABLE_RDD_SANDBOX = 1;
     LIBVA_DRIVER_NAME = "nvidia";
     NVD_BACKEND = "direct";
-      MOZ_DISABLE_RDD_SANDBOX = 1;
   };
+  envStr = concatStringsSep " " (mapAttrsToList (n: v: "${n}=${escapeShellArg v}") env);
+in
+{
+  programs.firefox = {
+    enable = true;
+    package = pkgs.firefox.overrideAttrs (old: {
+      buildCommand =
+        old.buildCommand
+        + ''
+          substituteInPlace $out/bin/firefox \
+            --replace "exec -a" ${escapeShellArg envStr}" exec -a"
+        '';
+    });
   };
-  programs.firefox.enable = true;
   home.persistence."/state".directories = [
     ".cache/mozilla"
     ".mozilla"