Some other folks were reporting (complaining) about avast blocking their gmail, and Thunderbird, a few weeks ago, in a posted thread titled, Security Certificate. That one was solved by simply uninstalling avast!, then installing the new updated version. That discussion might shed some light on your problem - take a look. It’s at the link following, top of 2nd page, reply #15. More info down through item #21.
http://forum.avast.com/index.php?topic=126642.0