Following up: I see that currently my privacy.window.maxInnerHeight = 900 and privacy.window.maxInnerWidth = 1400. What they were before FF 133.0 I couldn't say, except as noted above, the width and height seemed equal.

In that my screen resolution is less than the dimensions cited above, I doubt those setting have anything to do with the problem -- which remains after the 133.0.3 release.

Attached is a screenshot showing the magnitude of the problem. As I failed to mention originally, this is in fullscreen mode.

This is for privacy.resistFingerprinting?

The old values were 1000 and 1000 (according to the code diff for bug 1556002). I didn't see an explanation for changing these dimensions, except that the sizing was standardized in some way.

(A square aspect ratio doesn't correspond to any normal display, so I can't understand why it was ever set to 1000 x 1000 if you don't want to stand out as a unique user.)

"This is for privacy.resistFingerprinting?"

Yes.

"The old values were 1000 and 1000 (according to the code diff for bug 1556002). I didn't see an explanation for changing these dimensions, except that the sizing was standardized in some way."

In that any hard-coded values don't seem to take into account monitor resolution, I fail to see the purpose of them in the first place.

"(A square aspect ratio doesn't correspond to any normal display, so I can't understand why it was ever set to 1000 x 1000 if you don't want to stand out as a unique user.)"

Not that my display resolution (height) is even capable of 1000 x 1000. However, on the theory that some weird interaction was causing the narrower letterboxing, I just adjusted my settings to 1000 x 1000 -- with no effect! I suspect some other change is behind this.

I don't use RFP but my understanding is that it tries to make you blend into the crowd a bit more by standardizing certain measurements that might otherwise put you in a smaller group. Maybe you can get more info on the changes over on r/Firefox: https://www.reddit.com/r/FirefoxCSS/