The problem with Avast Passwords slowing or stopping Firefox with unresponsive scripts, needs to be looked at again. It appears the issue was never resolved. Is there even an update on this?
Firefox may display warnings about a slow script in the current version (1.1.3730) on some pages. This may happen for different reasons. A fix for pages containing a huge number of forms is a part of the upcoming release. If it doesn’t solve your problem, please post the problematic pages.