I've been fighting this for a few days. The platform is a windows XP SP3 domain client with Acrobat 6.0.6 and Acrobat Reader 9. (The platform was recently re-installed from scratch owing to malware infestation and is now "pristine".)
I discovered that if I install Acrobat Reader 8 PRIOR to installing Reader 9, PDF files will open in the browser windows. BUT, if Reader 8 is NOT installed prior to Reader 9, the PDF file won't open and a commonly reported error message occurs.
FireFox shows that when the viable installation sequence (as above) is followed, the Adobe Acrobat Plugins installed include support for mime types: pdf, vnd.adobe.pdfxml, vnd.adobe.x-mars, vnd.fdf, vnd.adobe.xfdf, vnd.adobe.xdp+xml, and vnd.adobe.xfd+xml. If Reader 8 is NOT installed prior to Reader 9, only the pdf mime type.
The PDFs that were the immediate cause of all this trouble were PDF forms with blanks to fill-in -- if that makes a difference.
The symptoms are reproducible.
Also, if you have Reader 9 installed and are experiencing this problem, simply uninstall it, install Reader 8, then install Reader 9. The required MIME type support seems to be supplied by the Reader 8 installation and stays around when Reader 9 is installed.
Hope this helps some folks.