The problem downloads are all PDFs?

To switch between automatically opening PDFs in the browser and having the option to download them first, check in the following two places:

(1) Firefox application preferences

Firefox > Preferences > Applications

In the search box, type pdf and pause until the list is filtered, you usually get two to four different items. You can use the drop-down list to change behavior here.

(2) Acrobat Internet preferences

Edit > Preferences > Internet (that's the location on Windows, not sure on Mac)

Uncheck the box to view PDFs in the web browser

Does that help?

Oh, just noticed there is now a related help article: PDF files are blank or cause an error message on Mac

This is great! I have tested it on three different sites, and so far it has worked on each. I guess I didn't realize that .aspx was the same thing as .pdf, so I wouldn't have thought to check. Oh, and (1) worked for my Mac.

To make a short story long, .aspx is a file extension that causes a page to be processed by the Microsoft ASP.Net processor on the server. So like other server-side file extensions that trigger special processing (e.g., .asp, .cf, .php, .pl, .py, .shtml and many others), you don't really know until the browser receives the page whether it is delivering HTML, a PDF download, or something else.

Actually, my point in mentioning that was simply that you wouldn't want to have Firefox always handle .aspx pages as PDFs because often they will be regular HTML pages.