backwards: Fix firefox binary paths

hosts/backwards/modules/firefox/default.nix

@@ -29,7 +29,7 @@ let
   ff-alex = pkgs.writeShellApplication {
     name = "ff-alex";
     text = ''
-      ${wrapped}/bin/firefox -P alex --new-window "$@"
+      ${wrapped}/bin/firefox-devedition -P alex --new-window "$@"
     '';
   };