Add `external_pdf` option to Firefox (and match Chrome's PDF behavior)

[mdmintz]

Add external_pdf option to Firefox (and match Chrome's PDF behavior)

With Chrome, if you navigate to a PDF URL, then the PDF is displayed in the browser, unless you set external_pdf to True, or use --external-pdf, in which case it will download the PDF directly to the "downloaded_files/" folder.

With Firefox, there's no external_pdf option yet (and the default behavior is to download the PDF).

With the incoming change for this ticket, Firefox's behavior will match Chrome's behavior. (The external_pdf option will be added to Firefox, and it will work the same way as with Chrome.) This will Change Firefox's current default PDF behavior, but it will be consistent with Chrome. ("pdfjs.disabled" is the internal Firefox preference that controls this feature.) After this change is made, if you want PDFs to get downloaded automatically when going to a PDF URL, set external_pdf=True / --external-pdf.

[mdmintz]

This was resolved in 4.31.3 - https://github.com/seleniumbase/SeleniumBase/releases/tag/v4.31.3