More correctly get home directory in Firefox config

modules/firefox/default.nix

@@ -1,8 +1,8 @@
-{ pkgs, user, ... }:
+{ pkgs, user, config, ... }:
 
 let
   startpageDir = import ../startpage/dir.nix;
-  startpage = "file:///home/${user}/${startpageDir}/index.html";
+  startpage = "file:///${config.home.homeDirectory}/${startpageDir}/index.html";
   firefox-csshacks = pkgs.fetchFromGitHub {
     owner = "MrOtherGuy";
     repo = "firefox-csshacks";