Tabs are not MDI
Dave Hyatt explains why Mozilla’s tabbed browsing is different to (and better than) Opera’s MDI model:
Tabs are not a replacement for window management. They are an enhancement that can be used in conjunction with the window manager of your operating system to group thematically related content in separate windows. Different topics can be kept in different windows, and then within a window you can have any number of pages open that are all topically linked. This is much more powerful than MDI, as it isn’t about replacing your window manager or keeping you inside a single browser window.
Dave observes that Opera have added this dual-behaviour to Opera 6, following on from Mozilla’s lead. On a related note, I am currently using a dual head graphics card with two monitors hooked up to my machine. This allows me to have a Mozilla instance (with tabs) running on both monitors at once. Lovely :)