I'm not really up to date on UI development, and realize the thin scrollbar issue on Firefox has been discussed before, e.g., in MDL-77803, but we think this one is not resolved yet.

Steps to reproduce

1. On Linux or Windows, start Firefox, and go to 4.4 Sandbox: https://sandbox.moodledemo.net/course/view.php?id=3
2. Log in as teacher
3. In edit mode, add enough blocks so that the block drawer scrollbar appears.
4. Observe that it is impossible to grab the block drawer scrollbar with the mouse.

This happens in 4.3 as well. We noticed this after updating to 4.3. from 4.2.