Fun fact: Opera had a tab functionality before Firefox. In fact a little-known browser called InternetWorks from the 90s is thought to be the first that had them.
I was an Opera user. They were the innovators in the browser space back in the day. Eventually it just felt too bloated, and sadly now they are essentially another Chromium fork.
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.
