Well, as far as I understand it, a
Tabbed Document Interface (TDI, Tabs) is usually seen as a variation of a
Multiple Document Interface (MDI).
K-Meleon up to version 1.1 has a
Single Document Interface (SDI). With the Layered Windows Plugin, several windows can be grouped together and are then layered above each other (except the layers bar and the displayed document, the window properties are all equal). In this sense, Layers are a simulation of a TDI for a SDI.
See also
here.