Why does opening a pdf with pdf.js sometimes leads to the "save as.." dialog instead?

Question

I've set firefox to my standard pdf viewer because I've had mostly good experiences with pdf.js. Unfortunately, some pdfs won't open and the "save as" dialog ins displayed instead. Why does this happen and how can I fix it?

Answer 1

Most likely because the server explicitly sends Content-Disposition: attachment.

There is no reasonable way to override that at the moment, unfortunately.