[Bug]: Vertical Tabs Issue

[FlameQuests]

Pre-Submission Checklist

• [X] I have checked the existing/closed issues to ensure this issue has not already been reported.
• [X] I am using the latest supported version of the browser.
• [X] I have tried disabling all browser extensions/add-ons to rule out interference from these.
• [X] I have tried clearing the browser cache and cookies.
• [X] I have checked with a new profile and the issue still occurs.
• [X] I have provided detailed step-by-step instructions on how to reproduce the issue.
• [X] I have included relevant screenshots or console outputs.
• [X] I have checked that this problem is not occurring in the latest version of Firefox ESR.
• [X] This issue is specific to this browser and does not occur in other browsers.

Bug Summary

With vertical tabs, the indicator for a tab being in a container (coloured line) does not show when the container tab is on the top.

https://github.com/user-attachments/assets/1f6d3709-7313-4a96-80a5-b474ae939e49

Reproduction Steps

1. Enable vertical tabs
2. Clear all tabs
3. Open up a tab in a container

Expected Behavior

A coloured line should show on top of the tab which is in a container

Actual Behavior

A coloured line does not show on top of the tab which is in a container

Screenshots

https://github.com/user-attachments/assets/7f89cccb-daaa-46b4-bd42-94bbfb6a13bc

Browser Version

A coloured line should show on top of the tab which is in a container

Operating System

Windows 11

Additional Context

No response