This depends on Tree Style Tab extension.

Did you have any experience losing the active tab in a scrollable sidebar when it is out of the visible area?
This provides a marker for the active tab over the scroll bar. If should help you to know where the active tab is at.

Install

• License: Mozilla Public License 2.0
• Weekly downloads: 5
• Average daily users: 368
• Rating: 3.7143/5 of 7 ratings
• Created: 2021-08-07 15:04:35
• Last updated: 2022-09-13 01:40:14
• Homepage:
• Support site and email
• Orig: https://addons.mozilla.org/en-US/firefox/addon/tst-active-tab-on-scroll-bar/
• API: tst-active-tab-on-scrollbar@piro.sakura.ne.jp