It seems like a Firefox glitch. I tried it out on my old (barely-functioning) phone and the status bar is in fact there, but it’s displaying white text on white background so the only thing I could see was the battery icon (thankfully it doesn’t override the colour of the orange battery-saver icon).
I think more precisions are needed here. What theme (dark/white) are you using? I remember having seen the same issue on my FP2, the fact that the status bar was not shown was in dark mode, because the status bar remained white because the forum has no dark mode, but the icons became white as if the status bar had become black (or something similar, I don’t use Firefox on mobile anymore).
Firefox hasn’t developed their PWA features to a mature level yet. But it’s also worth pointing out that the Fairphone forum’s custom theme is a bit glitchy as well. All Discourse forums I installed as a PWA with Firefox have thsi status bar issue you’re describing.