Here is how to stop firefox from force downloading PDF

connect.mozilla.org Re: Firefox now saves files that I only wanted to open

I just learned some good news: Firefox 103 users can switch the case (B) to case (A) using a setting. Could you make this change, turn off the extension(s), and test again? Here's how you can check on that: (A) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click...

Hey guys

Today I got so annyed by firefox's default behaviour of downloading each and every PDF file to my disk that I went searching for a solution until I had the problem fixed. And it seems like I have finally found it. I have linked the solution but here is the explanation in short:

Firefox determines what kind of file type it is based on the content-type header it receives from the server. Another header is the content-disposition header with which the server specifies how the file should be handled. The two most important options here are attachment and inline.

inline is the default if not otherwise specified, and means the browser will handle the file according to the behavior set in the browser settings.
attachment means to always download the file

It is therefore possible that some pdf files are downloaded by force and others are handled according to the behavior specified in the settings. To force the latter in any case, you can proceed as follows:

go to about:config
change browser.download.open_pdf_attachments_inline to true

Thank you jscher2000 for the solution!