Eventually, I succeeded in installing it and making it work using the flatpak Job but the instructions were incorrect/insufficient and I had to figure it out myself. First, flatpak --user install org.gabmus.giara.flatpak, but then it turns out

The application org.gabmus.giara depends on runtimes from:
https://nightly.gnome.org/repo/
Configure this as new remote 'gnome-nightly' [Y/n]:

I say yea, and get an error:

error: The application org.gabmus.giara/x86_64/master requires the runtime org.gnome.Platform/x86_64/44 which was not found

so then I try

flatpak remote-add --if-not-exists gnome-nightly https://nightly.gnome.org/repo/

and get

error: Signature made sön 11 jun 2023 21:05:40 using RSA key ID 6A7C5D4482170E3D
Can't check signature: public key not found

Turns out the flatpak repos had moved:

flatpak remote-add --if-not-exists gnome-nightly https://nightly.gnome.org/gnome-nightly.flatpakrepo

worked, and then

flatpak --user install org.gabmus.giara.flatpak

and finally

flatpak run org.gabmus.giara

And then logging in actually worked, it took me to the page on Firefox, I pushed the button, Firefox asked me if I wanted to let the external app open it, I did, and boom, it started working. Very nice design, looking forward to exploring it.

In reply to @veilblight:matrix.im
I've been having an issue with the latest version of Feeds (Flatpak). Some articles are showing overly large left-pane sections.

In reply to @veilblight:matrix.im
No, problem. Great app otherwise and much appreciated!