Works fine for the most part (Firefox Quantum 58.0.2 64bit), except these bugs:
- requires first-time manual window resize when opening a new tab
- doesn't properly resize when maximizing, have to do manual resize when un-maximizing
(can be kind of worked around by resizing the window to rough screen size, not practical though)

It does automatically switch to theater mode, but it doesn't make the video player bigger to fit the screen, it's still the same size as without this addon

Ruined youtube layout after uninstall. Now every time I load a youtube page it buonces between two sizes.

Totaly agree with Hillside,never seen it before,works like a charm,been forced to use chrome and improvedtube,great

خیلی خوب بود

"Use all available space in your browser window to display the video as large as possible."
Nope: http://imgur.com/a/TCH1o
Too big for my second screen.

I guess your browser window size detection technique doesn't work that well.

Perfect

I am the first to review. Possibly the most important ability to make tube video as big as possible only one Firefox addon has accomplished up until now in Youtube plus! Youtube plus conflicts with turn off the lights addon and this one does'nt! Great job.