Firefox (especially Gmail) cannot complete some requests: no responses are returned (zero bytes), assume that because of Endpoint Agent

It is almost impossible to use Gmail ( in Firefox. There are no problems in Chrome on the same machine.

In the browser developer console / Network tab I see some requests are without responses (zero bytes responses). Some with SSL_ERROR_BAD_MAC_ALERT

GMail app constantly shows yellow warning: "Ups... the system encountered a problem. Retrying in ..." .  "Retry now" link doesn't help.

Similar errors discussed recently:

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