DavidR
2
Web Rep is to very old name for the Avast Online Security add-on.
What version of firefox are you using ?
Firefox had plans to disable all unsigned add-ons in FF 42.0 (if memory serves me right), whilst it may be disabled in your FF version It was available on my other system win7 ff 41.0. That said Mozilla have cancelled the planned disablement of unsigned add-ons.
In about:config check the xpinstall.signatures.required option it should be Status = ‘default’ Type = ‘boolean’ Value = ‘false’ see attached image.