I could be wrong but it looks like Opera introduced tabs in 2009 with v10.5 while Firefox's first release was in 2002. Unless we're referring to different things.
Some people apparently think tabs has to be drag-and-droppable, which Opera had in 2003, in order to be called tabs: https://allthatiswrong.wordpress.com/2011/01/14/opera-did-no...