Firefox - "Error during XSLT transformation: Evaluating an invalid expression."All down to a double equals in an expression. Something I regularly make the mistake of doing, but under normal cercumstances, is easy to spot via xsltproc:
XPath error : Invalid expression
$leftspan == 2
compilation error: file xxx-xxxxx.xsl line 193 element if
xsl:if : could not compile test expression '$leftspan == 2'