Reload the PDF and make sure it's displaying "Save to Zotero (JSTOR)". It looks like in this case it's trying to save the page as an attachment directly, and that's failing. It shouldn't, but JSTOR sessions are a little weird due to their terms & conditions screen, which is what Zotero is hitting in this case.
(Also, just to note, it's usually better to save from an article page when one is readily available, but JSTOR happens to have a clear URL pattern, so the translator knows what you're looking at and can work properly even on when directly viewing a PDF.)
Upgrade Storage
https://www.jstor.org/stable/pdf/4105001.pdf
Whereas I can save just fine with many other PDFs I'm viewing in Firefox.
Debug ID is D2084839513
(Also, just to note, it's usually better to save from an article page when one is readily available, but JSTOR happens to have a clear URL pattern, so the translator knows what you're looking at and can work properly even on when directly viewing a PDF.)