Ok, if you really want the extension in Firefox, I found a Fix.

1 - Search for firefox 26 and Download it.
2 - Uninstall firefox but choose to keep all your data.
3 - Install v26.
4 - Let it open, close it. And open it again.
5 - Make your changes to Avast extension.
6 - Update firefox to the latest version.

Keep in mind that when you update firefox, you cannot access avast extension settings again. But they will be saved from the previous version.