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

Could you try disabling DNS over HTTPS from within FireFox as a test? 

1. Go to Settings > General, then scroll down to Network Settings and click the Settings button on the right.
2. Scroll down and uncheck the Enable DNS over HTTPS box and click OK.
3. Restart your browser to make sure that the new settings are applied.

My employer opened a support ticket. After enabling verbose logging I see the problem in the SophosNetFilter.log related to SSL.

Everything exactly as in: community.sophos.com/.../sec_error_reused_issuer_and_serial-error-when-using-decrypt-https-websites-using-ssl-tls-in-eap-using-firefox

Hi, do you find a solution? We have the same problem with Firefox and Sophos InterceptX.

Hello Qoosh, i have tested your proposal but the "Enable DNS over HTTPS" box is already unchecked in Firefox. We have the same problems but we dont use SSL/TLS decryption at the moment.

If the issue continues to persist, could you turn off the "Sophos Network Threat Protection" component through the Sophos UI? 

The NTP component will be involved in all of the web interceptions that occur on the endpoint so I only recommend doing so temporarily, or where absolutely necessary.

There is an update which will be released this month to address a related issue. This update will hopefully resolve the problem discussed in this thread as well. I will let you know here if I receive a more definitive time frame. 

Sophos AV is interfering with all components of google apps (worlds largest email platform - gmail) with the 3rd most used browser (firefox, 2nd largest if you dis-include safari) and it's been a couple of weeks now with no fix. I've tried various browser tweaks, created ticket with my firewall vendor and the problem when away by entering the tamper protection password and disabling Sophos. So what's the solution? Right now it's disabling Sophos, but what good is Sophos if it's disabled right?

Do you find disabling SSL/TLS decryption helps?  If so, that is probably the safest config change as it means you can leave Network Threat Protection enabled, web protection and web control enabled. Domains accessed will still be looked up using the SNI.  I suppose it all depends on if disable decryption helps?  As a check on the endpoint

HKEY_LOCAL_MACHINE\SOFTWARE\Sophos\Management\Policy\ThreatProtection\[latest revision]\web_protection

https_decrypt_enabled = 1 or 0

Along with changing DNS settings, some additional guidance related to firefox has been shared in the following document. 
- SSL/TLS decryption of HTTPS websites

1. Enter 'about:config' in the address bar and press Enter.

   A warning page may appear. Click Accept the Risk and Continue to go to the about:config page.

2. Set 'security.enterprise_roots.enabled' to True.

   This tells Firefox to trust the Windows root certificate store.

If your issue continues to persist, I recommend opening a case with Sophos Support so that we can look into the issue further and make compatibility changes for the future. 

Hello, I done this Change but it doesent helped me.

Do you have another idea?

Someone (Mozilla forum) suggest to change security.tls.enable_0rtt_data = false on the about:config in firefox config. Let's give a try.