When watching a flash player video, firefox crashes

PC specs: Windows 10 x64 x86
Kaspersky Endpoint Security
Intel Core i7 8650U
Intel UHD Graphics 620 128MB

Tested on versions:
Nightly 72.0a1
Beta 71.0
Release 70.01

Affected platforms:

• Windows 10 is only affected on nightly.
• On Beta and release it does not crash.
• MacOs and Ubuntu are not affected.

Steps to reproduce:

• Install flash player
• go to http://bear71.nfb.ca/
• play video, then try to resize window, or move firefox window after some seconds

Actual result:

• FF window hangs, sometimes it shows white screen when window is moved , sometimes white screen when there is a resize of the window.
• It ends up crashing 80% of the time.
• There was also one time where it showed a windows10 message saying firefox was blocked by windows (see screenshot attached)

Expected result:
Firefox should show the video with no issues, like it does on Beta and Release.

about:support, about:Crashes and link to crash are attached.

Will return with regression ASAP.

Notes: I tried running mozregression for this but it was all green.
The reason is because it not always crashes when playing the video. Sometimes it starts to play and there are no issues and sometimes the FF window just freezes and stops responding some seconds after the video starts to play. So there is no sure way to track the build that is causing problems because it might happen or it might not.

The priority flag is not set for this bug.
:jimm, could you have a look please?

For more information, please visit auto_nag documentation.

Bugbug thinks this bug is a regression, but please revert this change in case of error.