The ridiculous Firefox/Opera rivalry (it’s software, not religion) has given rise to one annoyingly persistent meme: the belief that tabs are just MDI (Multiple Document Interface).
MDI, as implemented in many Windows applications and eventually abandoned by Microsoft, involves having a mini-desktop inside your application, with its own windows that you can minimize, maximize, and rearrange. If you have a taskbar-like interface it can look a lot like tabs, and you can certainly use it the same way as tabs, but it’s a different UI paradigm.
A tabbed interface is very specific. You have only one view at a time in your application window, and you switch between them based on a row (or column) of tabs along the window’s edge. You can look at it as a proper subset of MDI, but it is not the same thing.
Additionally, classical MDI uses one master window for the application. All documents appear in that window. Tabbed interfaces often (though not always) allow you to have more than one window, each with its own set of tabs. This makes it possible to group documents, web pages, etc. by category in a way that you can’t with a single-document interface or classic MDI.
So when people claim Opera had tabs first, they’re thinking of MDI—which Opera did have before Mozilla did. Tabs were showing up in browsers like Netcaptor and Galeon, however, long before they showed up in the Mozilla suite—and long before Opera hid its MDI capabilities under a tab-like veneer.
(reposted from Spread Firefox in response to Asa Dotzler’s post on the history of tabbed browsing)