Subtitle and caption support for many sites in Firefox's Picture-in-Picture player window is targeted to ship in Firefox 100.
Hi! If you're reading this, then you've asked me about or otherwise expressed interest in supporting captions / subtitles in Firefox's Picture-in-Picture feature.
As of this writing, Firefox does not currently support captions or subtitles in the player window. This is because most sites have invented their own mechanisms for display captions or subtitles. YouTube's captions, for example, are implemented in a completely different way from Netflix's.
This makes it rather difficult for us as a browser to develop a general solution to the subtitles / caption problem.
Difficult, but not impossible. We're currently working on ways for us to develop site-specific behaviours for Picture-in-Picture. This allows Firefox's Picture-in-Picture feature to be smarter in how it interacts with specific video sites. In theory, this would allow us to detect the subtitles and captions and display them in the player window.
We're still working on getting those site-specific adapters working. After that, we will need to apply effort in order to detect, clone and react to captions and subtitles, for each popular video site.
So we're well aware that people want this feature. It's just more difficult to pull off than it might sound at first glance.
You can follow this bug if you'd like to know when captions and subtitles will work in Picture-in-Picture.
Would love to know if this interesting comment on exploiting the full-screen custom UI has come to your attention