I have learned a long time ago not to make claims about how hard it is to implement something in other people's projects - I get it wrong often enough about my own.
(Also, I don't think there's a single "PWA spec".)
Firefox's UI can be easily tweaked with CSS. It's trivial, for example, to get rid of all chrome. The real problem is their ideological stance against PWAs.
It's definitely doable to make the bar disappear with a tiny bit of CSS. Did it myself in the past and the Firefox team does it partially for their vertical tabs feature
(Also, I don't think there's a single "PWA spec".)