Avast Mac Security breaks Firefox and Thunderbird Certificates

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When I install Free version of Avast Mac Security on my Macbook with OSX 10.10.2 (Yosemite), Firefox 36.0.1, and Thunderbird 31.5.0 (latest versions of all of these), it breaks the certificate portion of Firefox and Thunderbird.

With it installed, Firefox does not allow going to many websites (google.com, yahoo.com, avast.com, ...) and gives a message indicating there is a problem with the Certificate.  And, only presents a "get out of here" box to click on with no way to accept the site's certificate.  In examining Safari's certificates with Avast installed, I find that all of these sites use Avast for their certificate's Authority, yet without Avast installed they show different authorities.  On some sites, it allows you to accept the certificate but puts up a message for every site I visit which uses https.

As for Thunderbird, with Avast Mac Security installed, it complains about the certificate on each startup but allows you to accept the certificate.

If I shut these programs down and run Avast's Uninstaller then restart them without any virus protection, they work normally.

I've tried searching through the Forums for clues to fix this problem but not yet found anything helpful.  One person mentioned possible corruption of cert8.db which I can't seem to get to using Finder, but found using Terminal and confirmed its content is not changed when Avast is installed.

[tumic]

In short - the solution should be to close firefox (and thunderbird) and to do a user logout (or restart).
The apropriate "Avast trusted CA" certificate used for HTTPS scanning should then be installed in
Firefox/Thunderbird. You should generally not accept any "Avast untrusted CA" signed sites/mail servers
when secured connection scanning is enabled.

If the logout/login process does not solve the issue, then we would be interrested in the avast* debug
logs located in the /tmp directory so we can analyze the issue.

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Avast won't allow you to install it while either of those programs are running.  Plus, I rebooted while Avast was installed and same symptom after reboot.

At this point, I'm no longer looking for a solution for now as I "bit the bullet" and purchased a license to use Kaspersky for 1 year.  Once that license expires, I may revisit using Avast.

However, while I had Avast installed, I did a full scan.  It came back showing my computer was clear of viruses.  However, after I installed Kaspersky, I did a full scan with it and it uncovered several .exe files that had arrived years ago as attachments to email message which contained viruses.  This kind of makes me wonder how good Avast is for protecting your system and others.  While those .exe files with viruses would be benign for my Mac, they could be serious for the Windows 7 virtual machine I have running on my Mac.