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 (mail.google.com) 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: https://community.sophos.com/sophos-central/f/discussions/133650/ssl_error_bad_mac_alert-pr_end_of_file_error