Start Firefox in Use Troubleshoot Mode in Firefox to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

Try to disable hardware acceleration.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

If disabling hardware acceleration works then check if there is an update available for your graphics display driver.

I had already used the safe mode to isolate to possibly Adobe Shockwave Flash. In retrospect it seems the act of restarting has more with problem correction than Flash. TBD

At your suggestion, I did disable the hardware acceleration. Updated the NVIDIA Quadro 540 driver. Found that Windows XP and NVIDIA had different opinions about what is latest. Used NVIDIAs.

I have a serious short term memory problem (my human brain). It seems initial driver date , before starting update, was newer than either windows version or NVIDIAs verson. Not sure.

Yesterday, I also went into about config and set some options to default, as opposed to user value set.

Yesterday I also changed mouse scroll increments from 1 to 4 lines

I will reply in a few dates about results

I did do some other things like rolling Shockwave flash back to v10.

I believe the problem was corrected by unchecking "Use hardware acceleration when available" . Problem has not recurred in past week . Before it would manifest several times per day on both Firefox and Thunderbird