debugging XSLT in Firefox

How do people debug their XSLT under Firefox.  I got something working
under xsltproc, but it fails occasionally on Firefox with the
following error message:

[Exception... "Component returned failure code: 0x80600004
[nsIXSLTProcessor.transformToFragment]" nsresult: "0x80600004
(<unknown>)" location: "JS frame :: <elided> :: anonymous :: line 155"
data: no]

which gives me no clue what happened.  Web searches on that error code
don't turn up much either.

Is there any way to get more useful error messages, or is there a more
verbose version of the same XSLT engine that Firefox uses that I can
run my stuff against?

thanks,
mike