Also note that fair use doesn't apply everywhere, and IIRC, the UK equivalent (fair dealing) is usually considered not to apply to software. It certainly doesn't apply to trademark law, so Mozilla could perfectly reasonably disallow the use of their trademarks with unofficial builds of Firefox.
Also note that fair use doesn't apply everywhere, and IIRC, the UK equivalent (fair dealing) is usually considered not to apply to software. It certainly doesn't apply to trademark law, so Mozilla could perfectly reasonably disallow the use of their trademarks with unofficial builds of Firefox.