virtual keyboard won't pop-up on firefox (wayland)

  1. login using wayland
  2. enable virtual keyboard on accessibility settings
  3. start firefox
  4. click in some input elements, no keyboard appears

Does this same fault occur when logged in on Zorin X11 Desktop, not Wayland?

No. On X it works. I tried

export MOZ_ENABLE_WAYLAND=1
firefox

to no avail.

I believe the on-screen keyboard in use is Onboard.
With your confirmation that it does work on X11, then the likeliest answer is that Gnomes Onboard software still does not work on Wayland. There are a great many years old bug reports filed on this issue.

I saw these 2 users reporting they got it working on Wayland, so I thought it could be an issue in Zorin somehow

Understandable. But considering it is the same gnome-shell (3.38) and same Wayland, I can see no way of it being exclusive to Zorin.
The question of why that export works for some people and not others... Is a good one.
I suspect that the version of python, mixed with which Java comes into play.
Users report this same issue from Gnome-shell 3.24 to Gnome40.