I'm sure I've used split view, like over a decade ago

I think it used to be done through an extension that would create a second ff window and resize the windows to be aligned. I only remember that it felt rather clunky

For those who don't read the article in its entirety and want to test the feature in current stable Firefox:

Split View can be enabled in Firefox 147 and Firefox 148 from about:config by setting the browser.tabs.splitView.enabled option to true

I tried it already and it works just normal like a splitview would work. I personally don't have a use case for it... yet. I probably rather would just open a second window instead.