Left sidebar of web IDE not scrollable on Firefox

The folder explorer sidebar of web IDE is not scrollable in Firefox. Scrollbars are not displayed. Example URL: https://dev.gitlab.org/-/ide/project/gitlab/omnibus-gitlab/edit/master/ . That repo has way too many files than displayed in the screenshot (attached)

Firefox 59.0.2 on Debian Sid

@kushalpandya was also able to reproduce the issue.

Edited Apr 12, 2018 by Balasankar 'Balu' C