Tested with:

Release 88
Beta 89.0b14
Nightly 90.0a1 (2021-05-19)

Tested on:

Windows 10
Ubuntu 20.04

Steps:

1. Launch firefox and save few bookmarks (around 20).
2. Open the hamburger menu or toolbar library menu and open the bookmarks submenu.

Actual result:

The scrollbar is displayed with a small delay.

Expected result:

The scrollbar should be properly displayed without delay when opening the submenu.

Regression range:

https://bugzilla.mozilla.org/show_bug.cgi?id=1687828

Note: This is reproducible without proton enabled as well.
@Molly, could you take a look at this when you get a chance?

This is an intentional tradeoff that bug 1687828 makes. It's certainly not ideal, the ideal behavior would be to have the scrollbar animate in or out along with the rest of the view. But that's a lot harder to do, and I do think this is a bit better than what we were doing before, which was for scrollbars to briefly blink into existence during the animation and then go away again when the height of the panel changes.