Under Ubuntu. If an application is set to be the default file handler for a certain type of file, Firefox will allow you to open such a download directly in the application. To do this, it saves the file in a temporary location, and then hands over to the application. The problem is that the location firefox uses under Linux is /tmp and modern snap and flatpak desktop apps are not allowed to see /tmp. For instance, a .deb file is an installable software package. Ubuntu naturally uses the Software app to handle .deb files. But Software is a sandboxed snap app, and it won't open files in /tmp The workaround of saving files and opening them from the app works, but it defeats the purpose of having registered file handlers. And I do not know how to get firefox to save temporary download files somewhere else. I can't find any relevant configuration option. I tried editing the firefox launcher: env TMPDIR=/home/tim/tmp_firefox firefox but it still uses /tmp