When I am using the android Firefox browser to watch YouTube, it is fine when browsing and finding a video. I will click on a video just fine, and start watching it no problem. The issue occurs when I switch tabs or switch apps to do something else and return later. Once I return to the YouTube tab even if the video is not or was never in full screen, Firefox will then be forced into landscape orientation. This persists between tabs until I either restart the app or do a Google search.

I found a potential solution in these forums. In about:config there should be a setting called media.videocontrols.lock-video-orientation. I tried this, but there is no such setting there for me. I'm at a loss for what to do now.