It is a common myth, largely spread by Opera fanbois that Opera invented tabbed browsing. This is very simply, wrong. The first browser that can be said to have tabbed browsing was an Internet Explorer shell called NetCaptor, first released in 1997.
Out of the main browsers it is actually Firefox that can be credited with being the browser to mainstream tabbed browsing. Phoenix (what would later become Firefox) introduced tabbed browsing in September 2002.
Some people consider Opera to have been the first main browser to introduce tabbed browsing because of the inclusion of a Multiple Document Interface. This is incorrect and the difference is not trivial. An MDI is the interface Word 97 had for example. It allows many windows in one, but is not the same as a Tabbed Document Interface. A TDI allows for repositioning tabs on a tab bar, dragging and dropping between windows, open in new tab, easy switching between tabs without having to minimize etc. Fanbois will often cite Opera’s own version history as proof that they were the first browser to include tabbed browsing, however just because opera considered an MDI to be a TDI does not make it so.
When Phoenix 0.1 was introduced it had tabbed browsing support and if you use it today it will still feel quite familiar. Opera on the other hand still only had an MDI at version 6.10>, which was released October 31st, 2002. Opera did not get proper tab support until version 7 which was released January 28th, 2003 although the beta which included the same tab support was released November 13th 2002.
I hope that I have helped to clear this up and denounce this ridiculous myth. Anyone is welcome to see for themselves and download the older versions of browsers and they themselves can confirm that it wasn’t until version 7 that Opera can said to have gotten proper tab support.