How do I change Firefox to highlight "save as" or "open with" by default on the download dialog?

No, I am not asking how to set a default application or action.

Now that I have that out of the way, I want to know how to determine the default selected radio button for the download dialog:

I would like PDF files to have "Open With" default pre-selected, and everything else to have "Save File" pre-selected. The important thing is that I still want the download dialog to appear, but I don't like how firefox automatically selects one radio button or another (seemingly at random). Someone posted exactly what I am asking for (in much more detail) here: https://support.mozilla.org/en-US/questions/958705#answer-434781

As far as I am aware, Firefox determines what radio button select, but this cannot be changed by the user...

The default actions are determined via the file MimeTypes.rdf, so this file is the one that you should modify once to suite your preferences, then copy to new installations.

Here is an extract from the documentation :

Attributes

General

• RDF:about - identifier
• NC:description - the name of the file type that will be displayed on the download prompt

Choosing what to handle

• NC:fileExtension - file extensions to handle (note that content downloaded from the Internet will go by MIME type, not by file extension)
• NC:value - MIME type to handle

Choose how to handle it (absence of the following options means "download to disk")

• NC:path - path of the application to use to open this file
• NC:useSystemDefault - true (use the system default for this MIME type) or false (define a different action)
• NC:saveToDisk - true (default action is "download to disk") or false (don't download by default)

Additional options

• NC:alwaysAsk - true (always ask which action to perform) or false (don't ask which action to perform)
• NC:editable - true (entry can be modified with user interface) or false (entry is locked)
• NC:prettyName - the short (display) name of the application

The file should be placed in the Profile folder that is suitable for the operating system and distribution and Firefox version (or fork) that you are using.

The dialog itself will appear with the choices as determined by these settings, or you can set it to automatically execute.

Please note that if you use the same profile on all computers, then Firefox Sync is also a solution.

You cannot set the dialog box to appear but for a certain radio button to be checked. You can only change the default option.

You can do this within Options > Applications and then searching by the file type. See below: