What do you mean by 'just Firefox'? Firefox, Chrome, Edge, Safari all support Progressive Web Apps in general.
Older browsers, on the other hand, might not be able to use offline features, but at least you can use the app in the traditional always-on mode.
For your comparison with Java: It is true, Java is very device-independent, but the deployment procedures are much more complicated than simply entering a URL.
They are actually not really.
If you can't run a modern browser on your platform, you're out of luck.
I think the real "as much device-independent as it can get nowadays" is just Firefox
On the major platforms Java is as much as device independent as the web