Zotero 5: Firefox plugin can't save PDF open in Firefox built-in reader

Firefox has a built-in PDF reader: https://support.mozilla.org/en-US/kb/view-pdf-files-firefox

But if I open a PDF like http://www.rochelleterman.com/ir/sites/default/files/keohane1988.pdf in Firefox:
- the plugin incoreclty detect the PDF has a web page (certainly because Firefox use JS to read pdf)
- if I click the button to save the PDF nothings append.

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