The default web browser is Gnome Web aka Epiphany (not Firefox).
It is true that by default Firefox is also included (as at Byzantium?).
I think they should include only Gnome Web and you can install whatever other browsers you like (but they would have to fix PureOS Store so that it is easy to find and install Firefox, if that is still a bug).
I think the reason for including Firefox by default may be that the bugs are spread around across too many web pages - so for any given web page one web browser works better than the other and you need two web browsers to give a good chance that one or other web browser works for any given web site.
So a better(?) question might be: Why don’t they expend resources getting bugs out of Gnome Web?
… so that Gnome Web by itself has a good chance of working for any given web site. And you can still install whatever other browsers you like.
Also, why does my Librem 14’s version of Gnome Web work for almost everything I throw at it, but my Librem 5’s version of Gnome Web is junk and considered “insecure” and blocked by several common sites? Why are these two not convergent enough to be the same thing, simply differing by the x86_64/aarch64 target platform?
Did they already expend the necessary resources on this and forget to publish it to the arm version?