This is pretty cool, I just tried this for the first time. Thank you for the tip. https://www.youtube.com/watch?v=qY173FCff_c

Since it is a website feature, but I could not reproduce it not working, please try it with Firefox is Safe Mode to make sure it is not an add on. Diagnose Firefox issues using Troubleshoot Mode

I have tried Safe Mode. It doesn't help.

I have since discovered the YouTube High Definition Add-on, which permanently plays all YouTube videos in Theater Mode at highest resolution. Using it means I give up a heck of a lot of customizability, but bypasses the problem and gets me the larger view sizes I like.

The button to change back to non-theater mode is still nonfunctional, of course.

I checked the web console and sometimes it works in beta and sometimes it does not. However there was an error the one time that may be a hint. It was a mixed content error. I did not catch the mixed content icon, but you may? If not there is an about:config entry that is mentioned below:

Firefox blocks the insecure content and a shield icon will appear in the address bar.   See this article for more information about the new mixed content blocker: Mixed content blocking in Firefox

To temporarily allow the mixed content to be displayed:

• Click the shield icon in the address bar and choose Disable Protection on This Page from the dropdown menu.

To allow insecure content to be displayed in all secure pages, enter about:config in the address bar and double-click on this preference, to toggle it from true to false:

security.mixed_content.block_active_content

You can also use this add-on instead, to toggle the preference:

• Toggle Mixed active content add-on

You may want to do this only for the current session, then reset the preference back to "true" and check back with the site to see whether it's been fixed, since this is a global setting that exposes you on all sites, not just the one you care about.

Let us know if this helps!

Unfortunately, this did not help. There was no shield icon in my address bar when visiting Youtube, and manually toggling the block active content preference (and restarting Firefox without the HD Add-on) did not address the problem. The "Theater Mode" button continues to do nothing but change its own appearance.