I'm not a developer, so maybe I misunderstand what PiP is strictly speaking. But, I thought I had been using PiP for a couple of years now in Firefox. https://support.mozilla.org/en-US/kb/about-picture-picture-f...
Is this not PiP?
According to caniuse, it's a proprietary implementation that's only partially implemented [1].
Short answer: yes.
Here are some web platform features Chrome and Safari (desktop and mobile) are shipping but not Firefox:
* Container Style queries: https://web-platform-dx.github.io/web-features-explorer/feat...
* @scope: https://web-platform-dx.github.io/web-features-explorer/feat...
* Picture in Picture: https://web-platform-dx.github.io/web-features-explorer/feat...
* View Transitions: https://web-platform-dx.github.io/web-features-explorer/feat...
* Cross-document view transitions: https://web-platform-dx.github.io/web-features-explorer/feat...