Finally found a fix how to stop firefox downloading each PDF to your local drive - eviltoast
Re: Firefox now saves files that I only wanted to open
connect.mozilla.org
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 fix in short:

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

Thank you jscher2000 for the solution!

cross-posted from: https://lemmy.world/post/9785046