Affected versions

• 76.0, 77.0a1 (2020-05-03)

Affected platforms

• macOS 10.15.3,

Steps to reproduce

1. Launch Firefox and access about:preferences#privacy
2. Scroll down and click on the View Certificates button;
3. Click on the Authorities tab;
4. Scroll all the way down in the dialogue/window;

Expected result

• list is displayed properly;

Actual result

• one additional empty space bellow the list

Regression range

• not a regression, verified with older builds such as 43 and the additional row is displayed;

Additional notes

• attached screenshot with the issue;
• marking with the lowest severity since this might be something intended / rather than something unnoticed.

This is some kind of strange issue with the <tree> implementation, so I don't think we should spend energy fixing it.