Firefox, probably. Though Heroic Games Launcher is getting there real fast. And currently I very often use Baby Journal, though it's an app I wrote, so I'm not sure I can really call it "favorite", but it's definitely one of my most used FOSS apps currently.
Baby Journal could use an more detailed About page. It looks like it's a shared space for families to post/talk about their kids in digital saftey. What an awesome idea! Well done!