feat: "bake-in" firefox env vars

users/myuser/graphical/firefox.nix

@@ -1,13 +1,34 @@
 {
-  programs.firefox.enable = true;
-  home.sessionVariables = {
+  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;
-    # To allow vaapi access for hardware acceleration
+    # 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";
   };
+  envStr = concatStringsSep " " (mapAttrsToList (n: v: "${n}=${escapeShellArg v}") env);
+in {
+  programs.firefox.enable = true;
+  programs.firefox.package = pkgs.firefox.overrideAttrs (old: {
+    buildCommand =
+      old.buildCommand
+      + ''
+        substituteInPlace $out/bin/firefox \
+          --replace "exec -a" ${escapeShellArg envStr}" exec -a"
+      '';
+  });
+
+  # TODO userChrome
+  # TODO settings if they can coexist with non declarative stuff
 
   home.persistence."/state".directories = [
     "Downloads"