It comes with a library called APE (see [1]), which provides most POSIX-y things implemented in terms of Plan 9 native API:s. (For example, BSD socket operations are just functions wrapping the normal file reads and writes that you would do in native Plan 9 code.)

Some things do not work, though (permissions models are a bit different, and chroot is entirely unimplemented, for instance).

1: http://doc.cat-v.org/plan_9/4th_edition/papers/ape

That shouldn't prevent things like scripting languages or web browsers to be ported.

The difficult thing is getting it ported correctly. As several people have shown, it's easy to get Go running. However, getting Go running properly in such a way that you can continue to get updates from upstream is much more difficult. So you end up with ancient versions of gmake, gcc, python, mercurial, Go, etc., all mostly-functional but no longer getting bugfixes, because it's much more difficult to make a nice clean port that can be accepted into the upstream.

Oh, and given that the major web browsers all seem to be written in C++, and we don't have a C++ compiler... that's problematic.

Right. Even X11 got ported at some point, so there is not really a reason why it wouldn't work other than lack of interest.