Thanks for your advice. Here's what happened since ...
# run windows update incl driver update, but select first only any offered drivers to be updated
Windows Update wouldn't run - said the service wasn't running. Turns out there was previous
update that left it unable to run. The fix was to turn off automatic updates, reboot, then turn them
back on. Nine updates were installed but none were for drivers.
# device manager: delete all mouse/pad drivers, then let windows find a suitable one (scan for HW change)
The mouse driver didn't change.
# device manager: see if any device has problems, especially connectors to the mouse.
No problems show there.
# firefox: disable autoscrolling, disable smooth scrolling (until problem fixed)
# firefox: iron over the existing one a fresh install, just to make sure
Done
# go to the product manufacturer's website (HP) and find all drivers for this pc (you posted HP-Pavilion
AZ220AV-ABA HPE-150t, hence probably:
this one, but please double check), then start checking if you have them installed correctly, and if in doubt, start with the chipset drivers first, unless they seem very old, then try from the component manufacturer (e.g. intel, amd, realtek)
I let the HP Support Assistant check. It reported that we have the latest version of all drivers. Unfortunately it doesn't list them, so now I'll have to go back and check the hard way for obsolete ones.
# also run an sfc check
sfc /scannow found no problems.
In doing all the above I had no crashes, even though I was using FF. My son had more problems afterward.
First, a desktop window manager crash.
Second, a FF tab closed while showing a youtube video.
Third, there was a BSOD with the error "Paged fault in non-paged area", again while watching youtube.
I had him switch to IE, to see if that makes any difference.