According to their own blog [0], Opera implemented tabs in 2000. The earliest version of what became Firefox was released as "Phoenix" in 2002 [1]. Phoenix was renamed to Firebird in 2003 and again to Firefox in 2004 [2], in both cases due to name clashes.
Mozilla later added them after the concept became popular thanks to Firefox. Mozilla and Firefox browsers coexisted for quite a while and Firefox was the lite version of Mozilla that didn't include E-mail client and other such features that Mozilla did.
