Pinstripe will not install on Mozilla 1.3 final. From the Mozilla 1.3 release notes: “On Mac OS X, XPInstall has been disabled for 1.3. This means that extensions and themes will not install for Mozilla 1.3 on Mac OS X. (bug 196959)”.
Update (April 22, 2003): Mozilla 1.3.1 builds have been released which fixes the software installation mechanism on OS X!