Although XSLT 2.0 is not supported natively in Firefox, it is possible, via Saxon-B to perform XSLT 2.0, either as a regular user using the XSL Results extension, or within one's own extensions using the approach taken in its code (see below).
The extension author hopes to soon add support for having XSLT performed automatically when visiting a page containing a processing instruction for XSLT (and which was not already performed by Firefox's own 1.0 XSLT processor).
Also, perhaps due to lagging Java support on the Mac, the code currently does not work on the Mac. However, if someone familiar with Java can try compiling Saxon-B completely in Java 5 (rather than its current mixture of JDK 1.4 and 5, the Saxon-B library author suggests this might solve the problem--any Java developers would be most welcome to help me (the XSL Results extension author) fix this). (You can leave a message on the discussion page.)