There is a school of thought that prefers applications to delegate their window handling duties to the window manager, in order to have consistent behaviour acr

There is a school of thought that prefers applications to delegate their window handling duties to the window manager, in order to have consistent behaviour across applications. In that case, both MDI and tabbed UIs remove the capability of the WM to manage that specific rectangle of content, and require you to use in-app controls, which may not even exist. This is especially pronounced for those who use non-standard window managers, say Ion or Awesome window manager which let you tile new windows rather than having them appear whereever they choose. Dunno if that's jwz's reason, but it's one good reason that I've had in the past (although not for browsers, the number of tabs I keep grows too rapidly to manage nicely alongside long-lived windows)