Best (convenient) ways to containerise programmes?

So I’ve been using firejail all this time using the user namespaces feature, because archwiki page on firejail leads you to believe it is a desirable one. The user namespaces created by firejail are created by $USER, right? So, that shouldn’t be done right? I can guess this isn’t simply a yes/no question but I honestly don’t feel like looking too much into this right now.