FF video stream practically unuseable w/ 4.0.0

HHija,

  after upgrading 3.9.0 to 4.0.0, streaming videos in firefox becomes
unbearable. The frame rate went from something normal to now
near 1 fps: we can observe the screen scrolling down with new pictures.

This concerns only firefox (chromium is fine),
and it concerns only WQHD 2560x1440p (1920x1040p on second screen is fine),
and only when clicking the player to fullscreen (actual video
resolution or choice of streaming provider seem not to influence this).

Platform is FreeBSD 14.3, GPU is Intel Ivybridge HD Graphics 2500

Reference:
https://forums.freebsd.org/threads/firefox-streaming-framerate-is-horrible-since-2026q1.101468/

The culprit is clearly icewm: I repeatedly installed 3.9 and 4.0
during the running X session, and after clicking "Window Manager" in
the menu, the running video changes behaviour.

I found Bug Report "Variable Refresh Rate stopped working after
updating to version 4.0.0".
I have no idea what a Variable Refresh Rate is supposed to be, but
reverting the change mentioned in that report does indeed solve the
issue.
This issue is more dramatic than gets obvious from that report:
it makes icewm practically unuseable with video streaming at least
in my system configuration.

Sadly I cannot afford an international property lawyer, so I cannot
become customer of the Github Corp. to report there.

cheerio,
PMc