Just to round it off...
Updates were installed at boot up this AM and things seem to be OK.
Initially, it seemed a bit sluggish but Firefox is behaving itself and speed is pretty good.
As stated, I've never known it to slow down to the extent it did yesterday when updates were being downloaded/installed.
TBH, it didn't occur to me that that might have been the problem.
I really did think that the machine might have been on its last legs and, as I manage banking, credit card etc online, I was a bit worried.
Thanks to all who have commented and helped.
