YouTube MarkUp ✔️ Did I watch that? per TheNima

"YouTube MarkUp" is a Firefox extension that helps you keep track of the YouTube videos you've watched by adding a checkmark (✔️) next to video titles, without the need for login. The extension seamlessly marks videos during your browsing experience – whether you're on a watch page, browsing YouTube Shorts, tuning into live streams, or exploring playlists.

Key Benefits:

• Automatically marks watched videos on watch pages, YouTube Shorts, live streams, and playlists.

• Provides a clean, non-intrusive checkmark (✔️) for easy recognition of watched content.

• Enhances your browsing experience by preventing accidental replays and saving time.

This extension only uses local storage on your device to remember the videos you’ve watched. No personal data is collected or shared with any third party.