After enabling high cipher strength security on the Secure Client Gateway, connectivity to the HTML5 Gateway works from all browsers, except Firefox.
The error returned is:
Cannot communicate securely with peer: no common encryption algorithm(s). (Error code: ssl_error_no_cypher_overlap)
Connecting via Firefox using the highest cipher strength is not possible at the moment as the browser cannot connect to a gateway with such cipher settings.
Should Firefox access be required, lowering the Cipher strength on the Secure Client Gateway to medium would be recommended.