I am assuming your question relates to what is known as "tabbed browsing" and I believe that Microsoft didn't start using tabbed browsing until IE6 or IE7, but I am not sure my brain is properly remembering that, so forgive me if I am wrong.
Now this browser stuff goes back quite a few years and I am again not sure about my memory, but I think one of our ISOC founders was involved in the start of a tabbed browser back in maybe the 80s or 90s, but I am not sure.
As for the general question related to defining a modern browser --- that's tough. To me everything about the Net is modern because I remember computers that took up whole rooms and needed lots of air-conditioning and that was in the late 60s when I was at school and my school at that time had some of the best computers in the world.
I still have a few TRS80s, "Trash80s" we called them, and that is still modern to me. Maybe I can think of a true floppy disc as not "modern" any longer. I mean, the real floppy disc that could be bent. I still have a whole bunch of those, some not even out of the wrapper. But my TRS80s use those and I have quite a collection of all sorts of stuff on about 100 floppy discs. I think I can move those out of the description of "modern". But I still can't move that actual TRS80 unit out of the thinking that it is "modern" because I remember when they first came out and they were so small and so great, at that time.
Now browsers these days worry me, because I fear that all the hype about security is covering up the key point, commercialization. I suspect recent browsers are mainly so companies can do a better job of making money.
Sure there is security mixed into that, but I wonder how important the "making money" part of browser upgrade is, compared to the security? Maybe some day I can hire some people to do a proper study on that subject. Something like a percentage thing of 50% of the upgrade is for security and 50% is for commercialization. Some sort of way to easily understand what's going on.
A modern browser? What a difficult question? Especially for an old fart like me.