Some clarification:
If you are using a modern (Intel) Mac, there is no supported plugin for viewing PDF files within the browser window. (in Firefox - Safari of course can)
The PDF Download plug-in doesn't allow you to view PDFs in the browser - it converts them to HTML in the browser (or you can save them etc.)
PDF Browser Plug-in only works on PowerPC-based Macs:
http://plugindoc.mozdev.org/OSX.html#PDF
Further:
QUOTE
The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS X. If you are running OS X on a computer with a PowerPC processer, or if you are willing to run Firefox under Rosetta, then installing the Schubert|it PDF Browser Plugin will enable you to view PDF files within Firefox on OS X. There is currently no native plugin for modern Macintosh computers.
So, Tom - for you, it depends which of your machines you're using. Was the result you're seeing occurring on one of your PPC Macs? I just installed PDFKit on my G5 and it works fine. I haven't tried anything on my Intel MBP since Mozilla says in no uncertain terms all over the place that it just ain't possible.
(Someone needs to get busy on this!!)