Open with Zotero
Some webpages are such that the Zotero plug-in (in Firefox at least) cannot pick up the correct document while it is easy to just download the document via a button on the webpage. However, this makes things a bit cumbersome, since I have to (1) download, (2) locate the file, (3) move it into Zotero. It would be nice if I could just choose to open the file with Zotero in Firefox and it will be added automatically. However, opening with Zotero seems to do nothing currently.
But to be clear, the feature requested here isn't usually necessary even on sites without supported article pages. You want your browser set to display PDFs, not download them. If you click on a PDF and Firefox either saves the PDF to your downloads folder or displays it in the browser with a file:// URL, that's wrong — instead, set it to "Open in Firefox" in the Firefox preferences (the default), so that when you click on a PDF it's shown with an http:// URL. You can then use the Save to Zotero button to save the file directly to Zotero, which will then try to retrieve metadata for it. So if JSTOR weren't supported, you would just view the PDF in your browser and save from there.
This feature will really just make it easier to add files that end up in your filesystem other ways — say, from unsupported sites that also force downloading instead of viewing (which is fairly rare) or that you receive via a local email program (instead of webmail).
Regardless, on a supported site like JSTOR, clicking the save button on the article page (not the PDF) should save the metadata and download the PDF if you have access.