You can use it with the Firefox-based browser or download it from the Chrome Store and use it with Chrome-based browsers. No problem. It's been a while, I probably forgot to update it to be more comprehensive.

Edit: Added v6.0.2.5
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It is possible to make it centered. But it won't be automatic. Some people don't want it centered after zooming or moving. I think it might be possible to add a button or shortcut to do this. I'm busy right now, might add it in next month's update.

Yes, it is AI-Based Recognition. I use a pose-related model and estimate the pose from the frame. Once I know what the pose is, I rotate the video.

I can set the default when entering the watch page/fullscreen and zoom in/out according to the desired value. But I think that's what you really want. To make the subtitles appear, try using my extension called "Youtube Subtitle Position" first. It can move the subtitle position and has an option to remember that value. If it really doesn't work, send request via "issue/suggest" button on the options page and attach a link to the screenshot of the missing subtitle. I think it's strange that the subtitles are missing.

Thanks. There will be no subscription in this extension anymore and forever. I will think about creating something else instead.