Make Firefox open app without first prompting to choose from window with one choice

Question

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 application/vnd.openxmlformats-officedocument.presentationml.presentation.

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 I think this is a GNOME file manager used on older versions of Ubuntu and feels like an out-of-date (and strange) answer for systems without Nautilus installed.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.)

Answer 1

Files is just the name that Nautilus goes by these days.

Firefox has its own list of behaviours for opening files. See Preferences | General | Files and Applications (or something similar — I’m not using it in English).

When you open a file type for the first time you get a dialog box asking if you want to open or save the file and asking what program to use. If you check the box next to Do this automatically for file like this from now on then that program is stored in the list and you won’t be asked again.

If you’ve already opened the file type, it should appear in the list in Preferences and you can set the preferred program there.

Flatpak has advantages and disadvantages. If you really want to keep a program up-to-date without having to worry about PPAs, then it’s not a bad choice. The downside is that program will take longer to start and you may occasionally run into some issue with accessing files outside its sandbox (e.g. theming). The choices are still a little obscure for the ordinary user, but there is a GUI program called Flatseal (available on flathub & in Software?) that lets you tweak permissions of flatpak apps.