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
Is this Windows?
I assume this behaviour has to be related to web protection and control, is that the case?
If it's Windows, do you have a process called SophosNetFilter.exe? If so, you are using …
Someone (Mozilla forum) suggest to change security.tls.enable_0rtt_data = false on the about:config in firefox config. Let's give a try.
Setting security.tls.enable_0rtt_data = false fixed this for me. However, I have another device where this is set to TRUE and it has never experienced this problem!