That's what I'd already done but thanks.
The AVAST Firefox browser integration uninstall suggestion didn't work. What I mean is the uninstall worked fine, Firefox showed no AVAST extension being listed as an Addon on reboot. So I shut down Firefox again, used CCleaner to wipe history, cache etc and then re-installed the AVAST Firefox browser integration stuff.
No problems but on reboot no WebRep icon and when I went to Firefox Addons the same original message about it being unsigned.
BTW I am using AVAST v.8.0.1497. I know its old but my ageing laptop didn't like the last AVAST program update I tried and I had a job getting it back to this, IMHO, more attractive, unobtrusive version. I don't want to go through that again if it can be avoided.