How to Fix Firefox 65 Errors Caused by Antivirus Bug

Mozilla Firefox 65 is available for download right now on all supported platforms, and this new version comes with several major improvements, including some that enhance user privacy significantly.

But at the same time, it looks like this new release also causes browsing issues on certain Windows systems, and it’s all because of a compatibility bug hitting several antivirus solutions.

As reported earlier today, Firefox 65 fails to load many or even all websites on some devices where Avast antivirus is installed, with some reports also claiming that other security products may be affected as well, including AVG and Kaspersky.

Mozilla has already confirmed that there’s indeed an issue on Windows, and the company decided to stop the automatic rollout of Firefox 65 to prevent the problem from becoming more widespread.

In other words, Firefox 65 will no longer be offered automatically to Windows devices using the built-in updating engine, but users who want to install it will have to download the browser manually.

Little is known at this point about the bug, but it looks like it all comes down to some certificate issues, as antivirus products on Windows attempt to inject code into Firefox in order to ensure secure browsing.

The process fails in some instances, and this causes Firefox to be unable to load websites, with the application returning the following error:

Your connection is not secure. [URL] uses an invalid security certificate.
The certificate is not trusted because the issuer certificate is unknown.
The server might not be sending the appropriate intermediate certificates.
An additional root certificate may need to be imported.

While Mozilla says it is investigating, a workaround already exists, though there’s a chance it won’t be everyone’s cup of tea. Users need to disable HTTPS scanning in their antivirus solutions and to also adjust a setting in the about:config menu of the browser.

Disable HTTPS in Avast to resolve the bug

First and foremost, doing the HTTPS thing in Avast, is pretty easy because it all comes down to just a few clicks. Basically, follow the path below to uncheck the option called Enable HTTPS Scanning.

Avast > Settings > Protection > Core Shields > Configure Shield Settings > Web Shield > Enable HTTPS Scanning

At this point, HTTPS scanning should be off in Avast, so rebooting Firefox should now restore the full functionality of the browser. You can also reboot the system if you don’t see any difference.

However, some users on Mozilla’s forums suggested an extra workaround which involves enabling of a special flag in the browser.

To do this, launch Firefox and in the address bar type the following code:


You’ll see a warning that you’re about to change critical features of the browser, so just take it for granted. Don’t change anything beside what we detail here.

Search for a flag that uses the following name:


By default, this setting should be set to false, so just double-click it to set it to true. Restart the browser and check if the full functionality of the browser is back.

The Firefox flag you need to set to true

Mozilla says it is already working on a fix, but an ETA hasn’t been provided just yet. Needless to say, antivirus vendors are investigating the problem as well, yet it remains to be seen how fast patches would be provided.

What’s important to know is that this issue only affects Windows systems, but it’s not limited just to Windows 10. Windows 7 and 8.1 devices are encountering the same problem, so if you’re seeing the error detailed above, your antivirus product might be the culprit.

Leave a Reply

Your email address will not be published. Required fields are marked *