Unfortunately the container breaks certain extensions (e.g. keepassxc-browser). And that's ignoring the fact that, at least on my machine, the flatpak fonts look terrible for some reason and I often run into issues with the Gnome theme not being properly inherited.

I've been using the Debian packages supplied by Ubuntuzilla (despite their being hosted on gag Sourceforge): https://sourceforge.net/projects/ubuntuzilla/