r@vast, thanks for your interest, and reply.
No, didn't accidently click UI launcher as FF and Avast are separated at both ends of my taskbar.
I am putting this down to be an intermittent maybe one-off fault. I know such things are hard to reproduce. I hope the repair may have done some good. I will report if I see it happen again.
FYI, I did not rush in to open FF after startup, I did infact wait until the Avast icon appeared in taskbar, so should not have been caused by a race condition.