I always thought tabs were an Opera invention, or maybe one of those IE shells like Maxthon or Neoplanet? IE5:mac was a fantastic browser though and was widely regarded as having the best CSS1 conformance of its time, as well as some cool features like an eBay auction tracker sidebar that seem quaint today but were very useful to me at the time: https://alistapart.com/article/ie5mac/

On the flip side, the only time I can remember hearing an entire crowd boo Stebe Jovs was when he announced that IE 4.5 would ship as the default browser in Mac OS 8: https://youtu.be/WxOp5mBY9IY?t=142

Firefox (or the Mozilla browser that became Firefox) gave us our modern idea of tabbed browsing. If I remember correctly, it was initially an experimental feature buried in the settings, but it quickly became a popular mainstream feature that Safari and IE adopted.

Opera had an earlier version of tabbed browsing, but it was tacked on to their existing Windows MDI interface.

In the 90s, MS Visual Studio 6 had a 3rd-party plug-in that would add tabs to their existing MDI interface.

Borland's Delphi and C++ Builder may have been the first apps to provide a true tab-based document handling in the early-mid 90s.

IE5 for Mac was quite good. It handled transparent PNGs correctly around a decade before its Windows counterpart and was just generally more pleasant to develop for.

I always wondered why Tasman never got backported to Windows to act as a next-gen replacement for Trident.