system
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I somehow doubt you should expect to get much in the way of meaningful advice given the lack of detail in your post.
What operating system do you have, XP, Win 7, 32 bit, 64 bit etc
What version of Avast are you using (The program version is show in Help->About Avast. The current version is 2014.9.0.2008
Is it the free version, or one of the paid for versions. Is it an autoupdate, or a manual install over the top of an earlier version.
What version of Firefox are you using.
Have you tried a complete clean install?.
Avast 2014.9.0.2008 with all shields enabled works quite happily here on Windows 7 pro, 64 bit, with Firefox v25.0 with AOS extension 9.0.2006.53.