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.