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Finally found a fix how to stop firefox downloading each PDF to your local drive
(connect.mozilla.org)
A place to discuss the news and latest developments on the open-source browser Firefox
Iirc, I had to set that firefox doesn't open everything in the browser but downloads the pdfs to disk. It was in the settings.
I really didn't get the idea why I had to save pdfs manually. To me, it feels weird to not save pdfs automatically but I understand you if you rarely safe a pdf.