I got the text/xsl mime-type from trying to view your XSLT document on your server. It asks me to download it (claiming the mime-type is text/xsl.)
As unbelievable as this sounds, but w3schools is often trash when it comes to XML and such. There is no such mime-type as text/xsl. It is simply IE being stupid, once again. (Kind of like older versions of MSXML not supporting the correct XSLT namespace)
However, Mozilla does let you do type="text/xsl" in the processing instruction tag, because I believe IE requires it (it is so stupid!!).
However, the document MUST be served as text/xml, as per the XSLT 1.0 specification. Mozilla has strict enforcement of mime-types. Unlike IE, it doesn't do mime-type second-guessing or even overriding based on the extension, as file extensions are meaningless on the web.
I've never had any problems with client-side XSLT transformation in Mozilla when I'm doing everything correctly.