No posted answer in StackExchange worked: they are years-old and pertain to very old versions of Firefox.
EDIT: At first I posted about opening files from the downloads list. I later discovered I get the same "Open With" window when I choose the option to "Open with System Handler (default)" after clicking the download link.
I have Mozilla Firefox Flatpak v86.01. I want to open LibreOffice Impress for .pptx files. The mime type for these files appears to be
I get an "Open With" window asking me to choose an application. There is only one choice: LibreOffice Impress.
Why doesn't it just open the application instead of requiring me to hunt down the "Open With" window and double-click on the LibreOffice Impress icon?
xdg-open mydoc.pptx opens the file in Impress without a prompt.
Some old forum answers indicate that Firefox is using mime configuration files. Presumably, this part works since the "Open With" window shows the correct application.
$ grep vnd.openxmlformats-officedocument.presentationml.presentation /var/lib/flatpak/exports/share/applications/mimeinfo.cache application/vnd.openxmlformats-officedocument.presentationml.presentation=libreoffice-impress.desktop;org.libreoffice.LibreOffice.impress.desktop;
Two answers suggested doing something with Nautilus. I think this is a GNOME file manager used on older versions of Ubuntu and feels like an out-of-date (and strange) answer for systems without Nautilus installed. My GNOME file manager is simply called, Files.
flatpak is a package manager for sandboxed applications. Is this behavior a side effect of sandboxing? flatpak Firefox became available about a year ago and I assume my GNOME desktop Software app switched from preferring apps from Debian repositories to preferring apps from flathub. (firefox-esr version 78.8.0esr-1~deb10u1 is still installed on my system but not launched by GNOME desktop.)