Storybook: Firefox UI bug in addon list

Describe the bug
A clear and concise description of what the bug is.

The horizontal scrollbar is bigger than the Chrome one and covers the addon titles:

To Reproduce
Steps to reproduce the behavior:

1. Add multiple addons
2. Shrink the window until the addons panel is narrower than the addons list
3. See error

Expected behavior

To have a thin scrollbar link this one:

* How to fix it? *

Add scrollbar-width: thin; to the class in the following div:

Sadly, I don't know where that component is (I couldn't find it).

System:

• OS: [e.g. iOS, Windows10, MacOS]: Windows 10.
• Device: [e.g. iPhoneX, Macbook Pro 2018]: Desktop.
• Browser: [e.g. chrome, safari]: Firefox.
• Framework: [e.g. react, vue, angular]: react (but any?)
• Addons: [if relevant]
• Version: [e.g. 4.0.0] 5.0.11