A couple possible solutions:
1. Try running Firefox in safe mode, this will disable add-ons and can be used to see if an add-on is causing the problem. If it solves your problem see troubleshooting extensions and themes.
2. Try clicking on TOOLS, then OPTIONS, then MANAGE ADD ONS. Find the AVG Link Scanner AddOn, right click on it then chose DISABLE.
Firefox will tell you to re-start the browser, then all is well.
Go to AVG control panel (in the Windows taskbar, by the current time) and tell it to use the Link Scanner, and it should be good to go.
Also check out this site for some more great troubleshooting tips:
I hope this helps!
Windows Outreach Team