I've been reading about FlatPak for a while now in various places (Planet Fedora) but I hadn't given it a try yet. I saw Jiri's post on the planet earlier today and finally decided to install the Firefox Nightlies using FlatPak. Of course, it works really well. I've gone ahead and installed the Telegram nightly from the FlatPak website too.
The instructions are all there in the documentation here. It's really quite simple. On Fedora, first, you must have flatpak installed:
sudo dnf install flatpak
Then, you go to the FlatPak website and click on an app that you want to install. This opens up the Gnome Software centre that installs the application for you. The application then shows up in the list in the activities menu on Gnome. For Firefox, you can follow the instructions here. For example, I now have the Firefox nightly installed:
I now intend to make some time to learn more about FlatPak - I've read bits and pieces here and there about some of the great features it brings - sandboxing and so on - and it looks quite cool!