I just noticed this tonight. I’m using Firefox 2.0.0.3 and from what I can tell Firefox ignores xsl style sheets! 
Should I be bothered about this?
No! RSS is probably not the best thing to be styling when we consider the consumer and what they want. It is really meant to be grabbed, scanned and ditched. My attempt at pretty formatting is probably just crap to most RSS readers, machine and human alike.