The flatpak "app" system has always felt like amateurish techbro crap to me, but today I finally put my finger on why.
The website is loaded with how-tos for installing it & packaged applications ("apps"), but there's not even a hint of how to maintain, manage, remove, or update them.
It has a set of halfway decent manpages that *do* say how to do these things, but there's literally zero reference! You have to just know they exist, & you have to just know `man flatpak` is how you start.
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The only reason I use flatpak at all is because, in Google's tightly controlled walled garden of ChromeOS, it's literally the only way to install Firefox.
Mozilla Support, "Run Firefox on ChromeOS": https://support.mozilla.org/en-US/kb/run-firefox-chromeos
Okay, so there's a technicality. I could also install Firefox from the Android subsystem's Google Play app, but it would be the phone/tablet version, not a desktop/laptop version, which means the UX, UI, & add-on ecosystem are significantly restricted.