Just tested in a clean and fully up-to-date focal VM, and this works as expected: by default PDF files are opened in the built-in pdfjs viewer, but when I change the setting to use the system's default application, they are opened in evince.
Can you run the following command in a terminal and attach the file whose name gets printed? find ~/.mozilla/firefox -name handlers.json ** Changed in: firefox (Ubuntu) Status: New => Incomplete -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to firefox in Ubuntu. https://bugs.launchpad.net/bugs/1908049 Title: Firefox cannot open pdf with viewer (evince) Status in firefox package in Ubuntu: Incomplete Bug description: Description: Ubuntu 20.04.1 LTS Release: 20.04 Package: firefox 83.0+build2-0ubuntu0.20.04.1 firefox: Installiert: 83.0+build2-0ubuntu0.20.04.1 Installationskandidat: 83.0+build2-0ubuntu0.20.04.1 Versionstabelle: *** 83.0+build2-0ubuntu0.20.04.1 500 500 http://at.archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages 500 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages 100 /var/lib/dpkg/status 75.0+build3-0ubuntu1 500 500 http://at.archive.ubuntu.com/ubuntu focal/main amd64 Packages Expecting: Changing the Firefox PDF setting from "Open in Firefox" to "Use system default application" leads to opening pdf resources with evince. What happend instead: pdf resources are opend inside firefox with internal viewer To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1908049/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp