Tested with:

Release 88
Beta 89.0b10
Nightly 90.0a1 (2021-05-11)

Tested on:

Windows 10

Steps:

1. Launch firefox and add the search bar to the toolbar.
2. Input any random string/letter in the search bar in order to open the drop-down.

Actual result:

The bottom right corner of the search bar drop-down is broken (see the screenshot attached).

Expected result:

The bottom right corner of the search bar drop-down should be displayed correctly like the other drop-down corners.

Regression range:

https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=52a2586bc322ece5ebb877f5ac67697c54037e6a&tochange=f89e1e839debb71ee6a3f49fee146a0d03c6a537

Note: This is not reproducible on macOS or Ubuntu.

This is a graphical glitch due to the new cliprounded shadow, I remember Neil saying it's not trivial to make it look right, because we draw the border but Windows draws the shadow.
Indeed the same graphical defect is visible in context menus.

Shilpa, From the point of view of Search, I don't think it's a P2a because this happens only on the legacy search bar, not the Address Bar.
The legacy search bar is not enabled by default but must be customized in.
Additionally this graphical bug exists in every menu using this shadow, like context menus, as visible in bug 1705254. We should either address or accept the problem globally, than just for the search bar.

Bug 1708735 is looking into simplifications to the panels code so we may be able to draw our own shadow at a certain point.

Got it, thanks Marco!