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Firefox Nightly on Android currently supports Split Screen and "window" view.
If you don't mind daily updates (pun half-intended) and possible instability, then you could download it.
Otherwise, there's the unfortunate risk that this feature will be kept locked in Nightly for an indeterminate amount of time, along with other features such as "Pull Down to Refresh" or about:config...
Does this include the ability to open a new window? I know in Chrome, Chromium, and Bromite for Android you can open a new window. This is helpful for streaming sites that allow you to open the chat in a new window, roll20, and a whole host of sites that work best when split into multiple windows rather than tabs.
+1 for implementing this. It's more than just split view, but the ability to run multiple windows of the same Firefox app. Ideally, this includes being able to open and move tabs in new windows. Currently, Firefox does not have this feature for Android Foldables or iPads, but Chromium-based browsers do.