For the past month I've been using Firefox Nightly as a flatpak, and I've been surprised at how usable it is now. There's a couple quirks:

• When you download something to a location, opening that location shows the path in a way that's not human-readable.

• Telling firefox to open that download location opens that same directory which doesn't show other files in the download location

The files are all where you placed them, but users might not get that impression.

Other than that, nothing has made me feel like I need to break the sandbox or switch back to the system package.