Firefox is only installed on casper

Students trying to open the browser on derecho will get a command-not-found
error.

notebooks/diagnostics/cupid.ipynb

@@ -427,7 +427,7 @@
    "source": [
     "When this command finishes running, you will have a new directory: `examples/key_metrics/computed_notebooks/key_metrics/_build/html/`.\n",
     "The easiest way to view these files is to look at them via JupyterHub, although its HTML parsing is not the best.\n",
-    "You could also run firefox from derecho or casper"
+    "You could also run firefox from casper (note that the browser is not installed on derecho!):"
    ]
   },
   {