Hi. I'm having the exact same problem.
My story: I live in Spain. My computer has Windows 7 and it had AVG Antivirus Free intalled. Last September 21st, AVG Antivirus got an update and, from that moment I couldn't get to open Firefox Browser. The program didn't even showed up, I only got the error message (0xc0000005) as deeverett said, but of course in Spanish. I unistalled Firefox and reinstalled it again, but the problem persited. So I downloaded and installed Google Chrome browser. Same as Firefox. It was downloaded and apparently installed without problem, but I couldn't open it. Exactly the same error message as Firefox.
After that I unistalled AVG Antivirus Free. And then both Firefox and Chrome opened and worked fine. No problems. I downloaded and intalled Avast Antivirus Free, and again, I wasn't able to open Firefox and Chrome. Exactly the same problem and error message.
Luckily I could do all that because I had MSN Explorer browser installed, and the problem didn't affect it. Explorer works fine with the new Avast/AVG Free Antivirus update. Your Avast browser is also working fine with Avast. I'm writing from there. Now I don't have AVG intalled.
I had been a user of Avast antivirus in my previous computer and was happy about it. But when I inherited my "new" computer it already had AVG Antivirus free installed, so I kept using that one. I'd like to keep both Avast and Firefox, so I would be very grateful if you could do something to fix that problem in the antivirus.
Finally, I don't know much about computers, but I managed to check the crash report folder of Firefox to look for any dump files. As deveerett said, no files in there.
Sorry, if all this information is not useful to you. Just wanted to help
Thank you very much.