From Wikipedia (I first saw it using Galeon. I'm probably messing up history here but I think Epiphany was also early with tabs):
The tabbed interface approach was then followed by the Internet Explorer shell NetCaptor in 1997. These were followed by a number of others like IBrowse in 1999, and Opera in 2000 (with the release of version 4 - although a MDI interface was supported before then), MultiViews October 2000, which changed its name into MultiZilla on 1 April 2001 (an extension for the Mozilla Application Suite[8]), Galeon in early 2001, Mozilla 0.9.5 in October 2001, Phoenix 0.1 (now Mozilla Firefox) in October 2002, Konqueror 3.1 in January 2003, and Safari in 2003. With the release of Internet Explorer 7 in 2006, all major web browsers featured a tabbed interface.
The tabbed interface approach was then followed by the Internet Explorer shell NetCaptor in 1997. These were followed by a number of others like IBrowse in 1999, and Opera in 2000 (with the release of version 4 - although a MDI interface was supported before then), MultiViews October 2000, which changed its name into MultiZilla on 1 April 2001 (an extension for the Mozilla Application Suite[8]), Galeon in early 2001, Mozilla 0.9.5 in October 2001, Phoenix 0.1 (now Mozilla Firefox) in October 2002, Konqueror 3.1 in January 2003, and Safari in 2003. With the release of Internet Explorer 7 in 2006, all major web browsers featured a tabbed interface.
https://en.wikipedia.org/wiki/Tab_(interface)