I'm asking this question mostly out of curiosity as it probably has little practical value.
I've noticed that if I change the user agent string for my Firefox browser using
general.useragent.override, fullscreen on YouTube no longer works. Initially, I thought it was because I used the useragent string of MS Edge which maybe somehow doesn't support it, but I've later used IE11's and verified that fullscreen works fine in that browser.
So, the current user agent I'm using is this:
Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko
This is what YT's toolbar looks like for me:
I'm using the latest version of Firefox -
44.0.2 at the moment of writing - on Windows 8.1 Pro x64. Of course, without the user agent override, this feature works properly.
What is the reason for this behavior?