Wikipedia lists Opera v4 having tabs in 2000, while they were added to Mozilla 0.9.5 in 2001: https://en.m.wikipedia.org/wiki/Tab_(interface)

Mozilla had multiple documents first, by just following Windows' MDI standard.

Then Netscape and IE got into a war for mindshare, and part of that was to ignore MDI and splash their browser windows all over the taskbar instead, to be more visible and grab more user attention.

Tabbed browsing was never a new invention, it was just a re-implementation of what we already had by way of MDI.

IIRC it was InternetWorks by BookLink Technologies

According to: https://www.makeuseof.com/tag/which-browser-invented-tabs-3-...

Opera also had tab groups, MRU tab switching, and saved sessions. Those exist in some form or fashion now, but the implementations are not as smooth.