I don’t think it’s possible. And please change the title of your post to something more descriptive. I just see it as “@mozilla”.

I believe even if you choose “Open in Firefox”, it will still download the PDF to the default download directory before opening it inside a Firefox tab. The behavior that OP describes above seems to prevent that downloading (and having the file around in your default download directory).

Obligatory question: Where is it? My guess is Scandinavia.