Long shot, if the video card is recent enough to have hardware acceleration in Firefox, maybe try disabling that .

EDIT: Re-read your message . Not what I thought initially. My suggestion does not apply, sorry .

Figured it out. As of FF 64 you cannot disable Notify about updates, it was removed from the Options menu.
"You can create a policies.json file and store that file into the
‘C:\Program Files\Mozilla Firefox\distribution’ folder.
Create a ‘distribution’ folder in \Program Files\Mozilla Firefox\ folder and place that
policies.json file into that folder with the following contents:

{
"policies":
{
"DisableAppUpdate": true
}
}

That's it."
NOTE: Above thanks to EP at Winaero
The "DisableAppUpdate": line can only be true or false. To restore Update notifications just delete the policies.json
Do not create and add the .json file (or delete it) with FF running.
That is all ! And it works ! Top picture is what happens every time I launched FF before the fix, and it happened at about 10 min intervals. A very annoying balloon popup. Bottom picture shows Options and updates disabled.