I found the answer myself. I turned off "Use recommended performance settings", the problem went away.

I think Firefox programmers need to delete this from the recommended performance settings because I can't imagine that anyone would like this feature. Also, I don't know what other useful features I may not have because this one feature is so annoying that I must turn off all recommended performance settings.

Hi danicaw,

I just marked your answer as the solution. We're glad to hear that you were able to resolve the issue and thank you for coming back here with a follow up. Your update could be really helpful to others facing the same issue.

This was not a solution. I thought it was at first but turned out it wasn't. I reported it to Bugzilla, others confirmed same problem with update 138, appears to be only in MS Windows 10. Fix should be coming.

Aha. I just undo the solution. Looks like this is bug1963666. Thanks for sharing an update!