Hi, I thinkI had the same problem Avast websheild was blocking https sites, google and microsoft and others for me. I turned off the "enable https scanning" under customise, and that seems to work withpout turning the whole webshield off.
That is definitely the preferred option rather than disable the web shield completely.
What operating system are you guys using ?
Using firefox 35.0.1:
Fortunately on XP this isn't an issue as the https scanning isn't available.
That said on my win7 netbook with https scanning enabled I'm able to connect to yahoo.com, which redirects me to uk.yahoo.com and google.com also appears to be working. I'm also able to connect to microsoft.com, that takes a little time to get started and then redirect to microsoft.com/en-gb/