hello, enter about:config into the firefox location bar (confirm the info message in case it shows up) & search for the preference named pdfjs.disabled. double-click it and change its value to true.

That's helpful, but not exactly what I was looking for. This makes the PDF automatically download, which is a definite improvement, but ideally I'd like to have the old open-or-download option back so I could decide on a case-by-case basis (and then if I choose "open," get it to open in Acrobat). Or, it would also help to know for sure that that's no longer an option.

Still, huge improvement (and I can definitely live with this if it turns out there's no way to do the thing I'm describing) -- thank you very much.

please also make sure that the default action for the entry named portable document format (pdf) in the preferences > applications tab is set to always ask. then it should work as intended...

(the screenshot is taken on windows but it will look similar on your system).

YES! It's working now. Thanks so much.