Recently released Avast Antivirus Free 2015 not playing well at all with Firefox, whenever you try to close or Exit Firefox, it crashes, the culprit here is the recently added ‘HTTPS Scanning’ feature in Web Shield.  To prevent Firefox from crashing, you’ve to disable HTTPS Scanning feature in Avast, here is how you can do that.

Fixing Firefox crashes due to Avast 2015

1. Open Avast interface > Settings > Active Protection, click on ‘Customize’ link in Web Shield

2. Once Avast Web Shield Settings window opens, remove the tick mark for ‘Enable HTTPS scanning’ and click ‘OK’ to apply the changes.

Avast web shield settings

3. Firefox browser won’t crash from now on whenever you close it when it is open.

Firefox 33, and Firefox 33.0.1 are affected (we don’t know about the older versions).

Note: Issue resides on Avast side, but not in Firefox.

Avast Users Fed Up with the Browser Cleanup Tool

Update October 31, 2014 : Avast has already resolved this issue from their end by delivering the fix through virus definitions, Firefox browser may not crash now and Mozilla to release Firefox 33.1 with a fix to this issue, so on both sides you’ve been covered.

Lukas from the Avast team in the Avast forum confirmed this and also detailed the bug summary.

 A summary of this bug:
– avast HTTPS scanning causes that all connections have valid (signed by avast root) but different certificate that is expected
– firefox uses certificate pinning to to verify the issuer of the certificate when checking extension updates
– eventhough the cert is “valid”, it is not good enough to validate the extension update connection
– when firefox sees this, the task is not aborted correctly and firefox crashes during shutdown on timeout

– fixed in Firefox 33.1 – will no longer crash
– fixed in Avast via VPS – URLs used for extension update check are not scanned – should not crash with Firefox 33.0 anymore

So you no longer need to turn off the HTTPS Scanning feature in Avast.

Does your Firefox have any issues with Avast? If you notice any, know us in the comments.