Hmm, that worked for me in i3 before but not in bspwm.

I tried FF PiP recently, but I have "bspwm config honor_size_hints true" and that makes the window resize itself until it's the size of a postage stamp.

The above is a known bug in FF, though apparently fixed in FF73: https://bugzilla.mozilla.org/show_bug.cgi?id=1600414