I'm on OSX Nightly as of today, and am scrolling down facebook debugging some other issue and am getting lots of

[Child 52104, Main Thread] ###!!! ASSERTION: Scrolling can't be reversed!: 'rightDelta > 0.0f && bottomDelta > 0.0f', file /Users/tritter/Documents/moz/mozilla-unified-2/layout/generic/nsGfxScrollFrame.cpp:1138
[Child 52104, Main Thread] ###!!! ASSERTION: Scrolling can't be reversed!: 'rightDelta > 0.0f && bottomDelta > 0.0f', file /Users/tritter/Documents/moz/mozilla-unified-2/layout/generic/nsGfxScrollFrame.cpp:1138
[Child 52104, Main Thread] ###!!! ASSERTION: Scrolling can't be reversed!: 'rightDelta > 0.0f && bottomDelta > 0.0f', file /Users/tritter/Documents/moz/mozilla-unified-2/layout/generic/nsGfxScrollFrame.cpp:1138
[Child 52104, Main Thread] ###!!! ASSERTION: Scrolling can't be reversed!: 'rightDelta > 0.0f && bottomDelta > 0.0f', file /Users/tritter/Documents/moz/mozilla-unified-2/layout/generic/nsGfxScrollFrame.cpp:1138

I'm not sure if this is indicative of a bug, or is a common occurrence and the assertion is superfluous.

What I can say as far as reproducing goes is I have a cruddy mouse and the mousewheel often scrolls down and then jerks backwards scroll-wise, so maybe it's related to very very fast scroll-down, scroll-up behavior.

I can reproduce this, using a current mozilla-central debug+opt build, just viewing my main feed at https://www.facebook.com/ (with a fresh Firefox profile, logged in to Facebook).

I don't even have to scroll; I simply load / reload the page, and I see two instances of that "can't be reversed" assertion.

I can reproduce using a debug build from ~7 months ago, too, launched via:
mozregression --build-type debug --launch 2020-06-01

Two instances of this assertion fire immediatley on pageload (after logging in). (And then my content process crashed with what looked like an unrelated (probably-fixed-by-now) issue in the JS engine.)

So: this assertion failure isn't from a recent change on our end, at least.

I see this same assertion a zillion times when visiting a TikTok user profile page, for what it is worth.