I have the same problem. Pages won't open (often, not always) but if I click Stop and then Refresh, they usually load instantly. The other problem which seems to be part of this is when they do load, many times they appear to be completely loaded but the page loaded icon at the end of my address window keeps spinning. If I don't click Stop, the page will eventually switch to a page not found or not accessible message even though it was OK. I'm using FF 27.0.1
The Avast Forum home page that's open on another tab has a spinning icon right now.....I'm waiting...now it has switched to "Problem loading page" on the tab and "Connection was reset" on the page itself although the page was fine before that. I haven't seen these problems when I temporarily disable Avast.
Someone needs to come up with a fix because with other good, free virus programs out there, it's quicker and easier to switch to another one than spending time searching for answers, posting at forums and waiting for an answer.