Add Subtitles — imadr

This Firefox extension let you add subtitles to any <video> element.

How to use
- Open the extension menu by clicking the toolbar icon.
- Choose a video element in the list.
- Choose a subtitle file with the file picker or upload from a URL.

Features
- Supports .srt and WebVTT files.
- Customize subtitles color, font and size.
- Adjust subtitle delay.
- Switch the video to fullscreen with the "Make video fullscreen" button to keep the subtitles in fullscreen mode.
- This extension doesn't work with videos inside an iframe, you must first right click the video then select "This Frame" > "Open Frame in New Tab"

Permissions
This extension requires the "Access your data for all websites" permission to be able to upload subtitles from any URL.