Why so many people here say it doesn't work or it is too difficult to use. Just do that, it's simple like a single click: https://cup.aihedy.com/fr/firefox/addon/open-in-pdf-viewer/#&gid=1&pid=2

It works well. When I right click a PDF link, it let me the choice to save locally or to open it in a Firefox page. But the thing that is missing is an option to have all the PDF opened in a Firefox tab, like Chrome does. And please, add the ability to open in a custom viewer, like portable Sumatra for instance.