Some time ago, the Crux developers in their wisdom decided to amalgamate the xulrunner and firefox packages (presumably because Crux doesn't officially include any other gecko-based browsers). Since then I haven't been able to build firefox on my fairly low-spec computer.

Well, I have finally found a solution and I'm posting it here in case anyone else has the same problem.

1) Go to the Mozilla site and download the latest firefox binary.
2) Unpack it (it's a bzipped tarball) and give the directory a suitable version-specific name such as firefox-9.
3) Working as root, move the directory into /usr/lib.
4) remove the link /usr/bin/firefox and recreate it to point to /usr/lib/firefox-9/firefox.

And that's it. After you have tested it, you can remove the /usr/lib/firefox directory that prt-get created.

The only disadvantage of doing it this way is that prt-get will still think you're using the old version so you must keep a lock on it to stop it trying to update.