Very often, websites are not programmed correctly. On these sites, when the user clicks on a PDF file, Firefox does not offer to display it in the user-specified PDF viewer.
Usually, this is due to an encoding error, a disposition error, a media type error, or another error (or a combination).
There are various Firefox extensions that fix one (or two) of these common programming errors, but I have yet to find one that fixes all of them, all the time.
Here's what I want: A simple Firefox extension that recognizes when the user is downloading a PDF file, and allows it to be viewed in the user-specified PDF viewer (or saved).
This could be accomplished via direct override, or by setting the correct encoding, disposition, and media type of any selected pdf file.
Note that there are some potential security issues with this approach, but this extension will be on a system that is never connected to any intranets and holds no important data.