The picture-in-picture feature is absolutely delightful and I love it, I used to split a specific tab off into a new window and rely on either Windows "snap" or a tiling window manager under Linux. However, because the Pic-in-Pic window is spawned as a new window, focused always-on-top, this means even when firefox isn't in focus, other windows will be placed behind it. This is logical and obviously the most sane development route. But it would be cool if there were some way to enable a "fixed to firefox" option, so that the always-on-top only applied to firefox windows, rather than the entire display environment.