I'm using Firefox 78.0.2 on Manjaro Linux and on Windows 10. The same happens for both OS. A few days ago (not at the latest update, but maybe the one before that?), the behaviour regarding PDF files changed. Before: When clicking on a link to a PDF file, it was opened in Firefox. When I downloaded it and then clicked on the file in the "Downloads" section, the local, downloaded file was opened in my system's PDF viewer. After: After downloading, when I click on the file in the "Downloads" section, the local, downloaded file is opened in Firefox again. The system default for opening PDF files has not changed, i.e., when I open a PDF file locally from the file system, it opens in the PDF viewer, and not in Firefox. Changing how PDF files should be opened in "Settings - Content" does change the behaviour for both cases simultaneously. Is there any way to change this back to the old behaviour?