The best alternatives to PiPiFire are Sideplayer, FloatPlayer, and PiP-it. If these 3 options don't work for you, we've listed a few more alternatives below.
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FloatPlayer is a simple Mac app that plays a floating YouTube-video on your screen. The player is visible even on top of full-screen apps. It's also completely open source. Check it out on GitHub.
iPhone users can finally use picture-in-picture with iOS 14, but the YouTube app blocks this feature. With PiP-it installed you can just tap Share > More > Picture in Picture with PiP-it from any video in YouTube and *boom* your video plays picture in picture.