When trying to view a PDF file in Preview on the Mac Firefox insists on downloading said file each and every time. Settings have been tweaked in application behavior in preferences and I have reset them several times by deleting the mime file in my profile folder.
Re: Firefox downloads ALL PDF's instead of opening
Sorry, I missed that you are on OS X 10.5.
I don't know what the best solution would be in your case.
I don't think that you can open the PDF files in Firefox, so you will have to download the file and open the file in Preview or another application.
Do you can an open with dialog to specify what to do with the PDF file?