Avast crashing Firefox

Hi.

Installed Avast last night plus the Firefox addon.
I use a news aggregator website called Newsnow - http://www.newsnow.co.uk/h/ - but since installing Avast and the Firefox add on I can no longer open links conventionally by left clicking inside Newsnow.
I have to right click links and open them in another tab. If I left click a link no new window opens and when I close Firefox, the process does not close properly and I have to kill the process in Process Explorer.
Disabling the Avast Firefox addon fixes this issue.