What in particular is it saying is infected?

If it is the program or installation file, if you downloaded it from Mozilla it will likely be a false positive.

I have been waiting for the message to pop up again, but it has not. It said it was apart of the main FireFox exe file. Yes I loaded from Mozilla web sit.

That will be a false positive, Avast may have updated their definitions to correct this which is why it has not popped up again.

Avast has update seen the last notice. So I think your right it was a false postive.

Thank you for your help.

Here is a screen shot of alert I am getting.