unable to switch Jupyter from Classical back to Modern Notebook in Firefox

[DrXyzzy]

1. Open any .ipynb file in Firefox (with jupyter rewrite).
2. Select Classical Notebook. (Bad idea, see jupyter python2 notebook may truncate in firefox #1537, but suppose you click by accident.)
3. Get error message:

   Unfortunately, Jupyter notebooks are not currently supported in SageMathCloud using Firefox.
   Please use Google Chrome or Safari.
   

4. Now try to open any other .ipynb file. Same error message.
5. Refreshing browser and restarting project do not eliminate the error.
6. Workaround is to open firefox about:config and set general.useragent.override to something else, like a Chrome user agent string:

   Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_4) AppleWebKit/537.36 (KHTML, like Gecko) 
   Chrome/58.0.3029.96 Safari/537.36
   

   refresh the browser, and select Modern Notebook for any .ipynb file.
7. Don't forget to reset general.useragent.override.

We probably don't want to display the Classical Notebook button in Firefox.