Right-click on the PDF file you want to open, mouse over Open With, and select Firefox. This will open the file in Firefox. You can also drag the file into a new tab on Firefox to open it.

On a side note, an open-source software I use called Sumatra PDF opens PDF files in a single window with tabs on the top for switching between files, and it also has a good search function.

Thank you for your reply. My problem is that what you describe does exactly not work. When I say "open with firefox" I am prompted with the pop-up window "what should firefox do with this file?" - and then choosing FF creates an empty "New Tab" and I am prompted again with the same pop-up. What differentiates FF PDF js from all other PDF viewers is, that it is an OPEN APPLICATION, which means I can search it with FF search tools across all tabs. Example: I do have 50 tabs open and I want to search for a specific term in one go over 50 tabs, I can do this with the add-on HUGO (I was the co-developer for this add-on, the author is Allasso Travesser) for tabs with html or PDFs that are loaded in FF PDF js Viewer. Now, with PDFs from the internet no problem, but I can't load my own PDFs into FF.