Greasy Fork

Hmmm....No...
I try too with another profile with less addons and scripts:
Same result.

The CSS selector of this button is .Pop-out.Video ?
I can't find it.
I didn't try with Quantum because they implement a similar thing for all videos players.

Tested on Chrome:
I don't see the pop-out button...
A screenshot?

It is very small and easy to miss. So I have added a screenshot, good idea!

Thanks: now i see it... on Chrome!
But Not in Waterfox Classic.

Have you tested your script with it?
Or maybe we need to be connected to Youtube?

The CSS selector for the button, is:
button[title="Pop-out Video"]
And i don't find it in Firebug....

Tested with Greasemonkey and Tampermonkey (same problem)

Now i can see it in Firebug!
But your button is added in an hidden container:
#info-contents[hidden]
If i make:
#info-contents[hidden] {
display: inline-block !important;
}

With that, the pop-out button became visible (but it need some CSS tweak around)... AND usable:
Work like a charm!

Note:
It seems YouTube serve an Alternate or New design to the Waterfox browsers:
That's not the same for Chrome or Quantum.

I saw that because broke suddenly one of my userstyle and i try to adapt it to these changes and hope to maintain its compatibility for the others browser.
After my tests it seems possible.
Damn Youtube!
Work in progress..

That is odd. Is the CSS rule that hides the #info-contents a @media query?

Let me know whether you plan to fix it at your end, or if you think it should be fixed in this script, for everyone.

Hmmm in a @media query? ?
Seems not.

And the "fix" i try is only usable for my userstyle.

If you can solve it on your side, it should be better.
That's because, with my first test, to only make your button visible, it seems touch other "hidden" things which need to be styled too...

But, if "fix" there is, it should not solve the "Waterfox problem" for every body...

Maybe if you can add your button in the control bar, i think it should be better.
By example near the YouTube Full Screen button, where we (me...) think it should be...

You are right, it makes more sense to go in the player controls.

Please give version 2.0.0 a try!

Yep!
That's cool.
The pop up take a little time to open with my poor Waterfox Classic,
but your script work perfectly.
;-)
Note:
I have seen your screenshot.
I hope it is useful too for an other user than me...

And i verified if it work with Waterfox Classic and without any CSS tweaks of me:
Yes!

👍 Version 2.02 should add the button more quickly

HoHo:
After a quick test, i saw 2 pop-out buttons.
One only should be enough no ?
:-)

That should not be possible. Please check if you have two copies of the script running. Maybe try to uninstall it/them.

Haha yes:
I forgot i tested your script with Tampermonkey too...
Solved.