I don't recall anything else you could try, you're returning the yellow-tint one and keeping the first one right?
Firefox has a declared (and open) issue regarding this:
https://bugzilla.mozilla.org/show_bug.cgi?id=1738926 and ppl is still having the problem on 12.1 as I experienced the other day and possibly yours too, happened to me with 12.0 and 12.0.1 as well. It is most likely created due to some issues w/ProMotion and high refresh display.
On Chrome there are also cases of some users complaining:
(can't find more links but I remember reading them)
And if you surf around, you can find a lot of people complaining of high WindowServer usage when using external monitors as well, on Big Sur and also on Monterey, so I think in your case both factors can be active at the same time, hence the high temps even on Safari. I see your memory percentages, but not CPU, my guess is if WindowServer is eating that much memory, CPU usage should be high as well.
Hope they mature Monterey enough on 12.2/12.3 and we're not facing so many issues on "normal use"
One last test I ask you, shutdown/reboot your machine, cause once WindowServer starts to eat CPU/RAM only way to stop is by reboot or killing WindowServer process which restarts the Finder too, then let it cool so temps start flat as shown, and in that moment, w/o connecting it to the external monitor, make the test playing same video with Safari, check Activity Monitor, do it on Firefox, and see if it creates again same issue, cause for me it wasn't 100% consistent to make the bug to appear.
Best of lucks!