Download Youtube CC failed on Firefox

[icarusxxy]

Summary

Download YouTube close captions / subtitles failed on Firefox, nothing happened after clicking the add-on.

Steps to Reproduce

1. On a YouTube video page
2. Click the add-on button

Expected Behavior

A toast messages saying "Subtitle copied to clipboard."

Actual Behavior

Nothing happened, clipboard content are the same.

Environment

• OS: Windows
• Browser: Firefox
• Version: 152.0.4