(*) Both Saxon and MSXSL do it in ~30 seconds, so it's really problem
with the XSLT performance in FF.

Perhaps it doesn't like things like:

<xsl:template match="text()[not(ancestor::artwork)]">

and [more inefficient XSLT]

Given rfc2629.xslt and rfc2616.xml, it's 24 s using Saxon 6.5, 15 s
using Saxon 9.0 and 45 s using LibXSLT 1.1.22. So as Andrew showed,
the problem is not with any particular engine, but with the inefficient
XSLT code.