Affected versions

• 88.0a1 (2021-03-07) , 87.0b5;

Affected platforms

• Windows 10, macOS 10.13

Steps to reproduce

1. Launch Firefox, access about:addons
2. Click on the Extensions page;
3. Click on the Cogwheel button in the Manage Your Extensions section (or any other section from about:addons);
4. Scroll up-down;

Expected result

• menu remains in view, page is scrolled;

Actual result

• menu is no longer displayed;

Regression range

• considered "good" builds that didn't close the menu, even though that implementation wasn't ideal (nothing moving behind menu when trying to scroll); so this range might not be as relevant this being a particular scenario;
• First bad: 2019-11-02;
• Last good: 2019-11-01;
• Pushlog: URL

Additional notes

• same if using bottom/side page scrollbars (hover&scroll not click scenarios);
• with a default browser size this is not as impactful, it becomes somewhat of an issue when the cogwheel and menu are scrolled out of view;
• attached recording to best illustrate the issue.