Not a Firefox support issue. Have you tried restarting your PC?

Have you tried Avast support?
http://www.avast.com/en-us/support

Or the Avast support forums?
http://forum.avast.com/

I have Firefox 21 and I updated my Avast 7 to Avast 8 and now I keep getting a stop error, and my shields are gone and a blue screen with stop error on it. Also I couldn't upload pictures to a site with FF but I could do it with Chrome. I checked with Avast and they are looking into also. Many people are being told it is Firefox and Avast 8 not compatible now. I don't want to change to Chrome but what else can I do? What would happen if I put a older version of Firefox back on. I think 19 worked great.

That is what I found out to be, FF 21 was not compatible with Avast 7 because I uninstalled the Avast twice as told in the help section before I got in touch with Avast before it worked with Avast 8, but it also installed the Chrome, which I always declined before when asked. This time I was not given a choice and installed, so I figured I had to have it, I will use it as long as Avast works since I have the paid version, but I still don't like it. Hope you have more luck and let us know if the older version of FF works for you with the Avast 8. I'm just tired of my computer not running the way I wanted.