Open links from flatpak via host firefox

I’m using flatpak for some applications, and have configured the xdg portal - I’m using the gtk portal. While I’m not using gnome, to my knowledge this should still work.

Whenever I open a link in a flatpak application, it instead prompts me to install an application via gnome software. According to

xdg-settings get default-web-browser

this should open with firefox.desktop.

What am I doing wrong? Is this an upstream bug? Should I be using gconf to configure the default browser? Is this a bug in our implementation of the portal configuration?