At first I didn't even realize this was FF feature (I suspected something like gstreamer, but was lazy to investigate). Now that I know it is indeed FF, I just cannot wrap my head around it...
1) I can pause the video, but cannot navigate it. I guess the navigation would be there if it was technically possible, but from practical standpoint this makes it useless even if it appealed to me in other ways. Popping a video into PiP mode and then go diving for the original tab to seek 10 seconds back to rewatch an interesting moment seems utterly bizarre.
2) I've grown to truly hate this kind-of-PiP thing many websites (news outlets mostly) do: Autoplay a video and then shove it to lower right corner when I try to scroll away. Along with timed "subscribe to newsletter" popup, this is an insta-close for me; Now why would I want to replicate this behavior elsewhere?
3) I'm literally unable to read through even the first words of that blogpost because of that gif. Now, thanks to this PiP feature, I can make any text unreadable by placing a distracting video on my screen.
While I'd welcome features that fight distraction (and not support attention fragmentation), there obviously is (significant part of?) FF userbase that will make use of this feature. Good for them.
Luckily for us grumpy farts, that blue PiP button can be turned off in preferences, we can mutter something about bloat, but generally go on with our lives.