It seems that the decision has already been made, but let me add a few
things I've been thinking, anyway:
A big feature of Fedora Next/Workstation initiative is stop gluing
things and start building something more coherent. To do so would
require not only Fedora contribute to the upstream as well as influence
it.
Therefore, since Fedora is in much better position to influence the
development of the Web (as well as GNOME as a whole) than Firefox, I
believe that preinstall Firefox on Workstation (making it a official
part of the whole), especially patched with some extensions, isn't the
best option.
In other words, at least for me, to put the focus in Firefox will not
help GNOME Web to get "ready", and will not help Fedora reach its goals.
Thanks for reading.