Windows - presume Apple & Linux. Doubt it is available or even useful on phones.

FWIW New to Edge (latest update) - Split Screen is on the … menu about ? of the way down.

Annnnnnnnd once alerted to this - Question: "FireFox"? Answer: been there for some time.

FF To enable
1) type "about:config" in the URL box.
2) type "split" in the search box
3) click the RH button till true appears for split.
4) restart FF
5) right click on an open Tab "Tab"
6) click split.

All browsers have/will have by now. Somewhere in there menu system.

Enjoy

PS the wheel mouse click on a hyperlink wheeze (try it) opens the link in a completely new tab (unsplit) so presume any hyperlink (eg with "target=_blank") that opens in a new tab anyway - behaves the same.