Fill Screen Anywhere — Zoom Into Videos by Farid Zelli

This extension allows you to zoom into videos across numerous streaming platforms.

On mobile:
Pinch-out on your screen (works anywhere) or long press the full screen button (YouTube only).
Note to IronFox users: Enable the "Allow for all sites" permission after installation.

On desktop:
Enter full screen and look for the "Fill Screen" button on the right side.
Keyboard shortcuts are supported.
See Manage extension shortcuts in Firefox for more info.

This lightweight add-on is fully open-source.