Avast Blocking Most Websites on Firefox

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I must turn off the Avast web shield in order to load almost any website on Firefox. The same sites load on Safari and Chrome just fine.

[Eddy]

What avast version ?
What FireFox version ?
What MAC OS version ?
Any message from avast or FireFox when a page is not loading ?

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I have the same problem:

Avast 11.5
Firefox 43.0.3
OS X 10.6.8

When trying to access yahoo.com, google.com, wikipedia.com, duckduckgo.com, etc. all give this error from Firefox:
   THIS CONNECTION IS UNTRUSTED

  TECHNICAL DETAILS

  duckduckgo.com 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.

  (Error code: sec_error_unknown_issuer)

If I disable Avast Web Shield, everything works. All these sites work in Safari 5.1.10.

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Forgot to say: my computer Date & Time are correct.

[Eddy]

This looks a lot like a problem with the SSL/TLS scanning option.
Definitely something for avast to have a look at.

To check, disable https scanning in the webshield and try to open those sites in FireFox.

[krahulik]

Hello,
  this can happen sometimes after a Firefox update. As a quick fix, try to logout and login again.

Best Regards,
  Martin

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I couldn't connect to www.my.yahoo.com so I ultimately Refreshed Firefox. (A mistake probably!) Then (and now) I was unable to open secured (https) websites. Some allowed me to override with an exception but others, including support.mozilla.org & facebook, wouldn't allow an exception. Finally I tried addressing the issue with Avast Mac Security, the free version, unclicking Scan Secured Connections in webshield preferences. That allowed me to access all the pages BUT I want the webshield protection. How can I fix the problem, reactivate Scan Secured Connections, and open the pages? Firefox preferences seems to show that I have a valid certificate for support.mozilla.org so that adds to my confusion. I just added support.mozilla.org to servers not needing to be scanned in Avast. That allowed the webpage to open BUT not with graphics. PLEASE HELP!

I'm using Avast Mac Security 2015, Version 11.7 (45814), on an iMac running El Capitan vers. 10.11.3, and Firefox 44.0.2. I have not had problems on Safari and Chrome seems to work pretty well.

THANKS MUCH!

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I have had to de-install Avast Free version under Mac os 10.12.5 because I could not reach a heap of websites that I normally use. The problem has already been described on this thread. I am using Firefox 54. I've wasted heaps of time on this. The problem is not evident on Safari. What is Avast doing?