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:
go to about:config
change browser.download.open_pdf_attachments_inline to true
Even with browser.download.open_pdf_attachments_inline set to false (default), Firefox already opens PDFs instead of downloading them, thanks to this setting:
For me it still downloads and then opens it in firefox. In the description it clearly says: Choose how Firefox handles the files you download from the web (...). And I don't want it to download the file to my disk in the first place just store it in chache like a regular website.
Which firefox version are you on? I'm on 122.0b1 (flatpak beta) and changing these settings just defines what happens after I download the file. Also my download folder is set to ~/Downloads