This has become one of the most frequent crashes (maybe even the top) so Firefox developers cannot ignore it. It has to do with Open h264 plugin installed on Firefox 33 and up, and Avast hooking into Firefox to scan SSL. Why Avast did not test this before they releases Avast 2015 is beyond me. The Open h264 plugin and Firefox 33 was released before Avast 2015, and the plugin itself was in the development channels for a very long time.
" Sounds a lot like Avast is doing an SSL MITM to scan stuff and we correctly" block that due to cert pinning (correctly because when we see a cert from a wrong issuer, we should block as it easily could be a real attack)."
"(In reply to Georg Fritzsche [:gfritzsche] from comment #35)
> Jorge, can you reach out to Avast on the breakage they cause?
I'll get in touch with our known contacts."
https://bugzilla.mozilla.org/show_bug.cgi?id=1087674