Confirmed FP Here: http://forum.avast.com/index.php?board=4.0
I just signed in to PayPal without any problems 
Just signing in might not be enough to trigger it. When making a purchase using PayPal there has to be something to handle the transaction from and back to the merchants store. That would be the area possibly at risk, though as I said I made a purchase using PayPal late last night.
@ johnchisholm
Whilst I don't use the Avast Online Security (AOS) add-on in firefox;
johnchisholm try disabling the AOS add-on in your browser and see if that works - if not then try the just disabling the Web Shield.
You said avast is blocking, do you actually mean alerting on it (as some others are reporting alerts) ?