The "avast online security" add-on doesn't seem to work with FF v27.
To be truthful the AOS has been nothing short of a headache.
What most have done is either disable the AOS in their browsers or
uninstall it completely. You can safely do either without compromising your
protection since the real protection is provided by the resident shields.
To uninstall...Control Panel>Uninstall a Program>double click "avast">click "change">untick "browser protection">reboot
FYI I don't have the AOS/Browser Protection installed at all. You get basically the
same thing by using
PrivDog. PrivDog is
much more stable than AOS.