No longer works, I am now getting black boxes on top and below of the video player while using theater mode.

To confirm I did reset my PC, as well as restart firefox to confirm nothing was missing.

Due to the recent Youtube layout changes, I was having trouble with the chat replays being displayed on the right side of the video and reducing the video screen size when playing LIVE streaming archived videos in theater mode, and then I found this add-on!

This add-on is very useful because it allows me to maximize the screen size of the video window without having to turn off the chat replay.
(Because turning off the chat replay function disables the “YouTube LiveChat Flusher” add-on, which I want to use at the same time.)

Unfortunately, with the version 4.0 update, the chat replay is now displayed to the right of the video and the video is not window maximized.
I would love to see this add-on become even better if the display method could be reverted back to the way it was before or if it could be made an option to choose how it is displayed.

I have now downgraded to version 3.9 and am using it.

This does what it says perfectly but if you are trying to get a boarderless window to watch youtube videos it is now natively supported by firefox https://support.mozilla.org/en-US/kb/turn-picture-picture-mode so I would just that instead of this. This does what it says it does though and is very similar.

Excellent, has a hotkey and doesn't interfere with other youtube addons.

Exactly what i was looking for. Thank you!

Small & works well. Good combination.

This extension is critical for anyone with even the slightest sign of OCD, as the space YouTube wastes in "Theater mode" is UNGODLY, and the fact that the video is just barely larger than regular mode is a joke. Massive Thank You to whoever made this.

It will be extreme helpful if have auto target apply while playing.👍

For superultrawide this is a must have.

Unfortunately not reliable. Often times it requires a refresh of the page for it to even work. The hotkey works 50/50 and it honestly fails more than it works on both chrome and firefox

This is so good! YouTube should take this feature officially!

Nice extension if you use a Ultra-Widescreen!

Developer was super quick to fix an issue!!

The issues that was:
Recently (about 2-3 days ago) I noticed that when using Windowed FullScreen, the video cut-off (expands to fill the area) instead of having black bars.