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

Getting the same issue as well, appears only in Firefox and majority seems to be Google sites like search, but it hit or miss as to when this occurs.

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.

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. 

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. 

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?