In other words, the ability to see two browser sessions, side-by-side, with a vertical split between them. Two viewports, each with their group of tabs. The same type of view you can get in, for example, Notepad++ with its "Tab>Move to Other View", or Visual Studio's "Tab>New Vertical Document Group".
I frequently arrive at situations where I want to compare the contents of one webpage against the contents of another webpage. So far, the most usable option I've found is to split the 2nd tab off into a new window, then arrange the two windows side-by-side.
There is "Side View"[1], but that shows a bare viewport, which makes browsing in the 2nd viewport much more restricted than regular browsing.
[1] https://blog.mozilla.org/en/products/firefox/its-a-new-firef...
Though the most innovative feauture is their deep integration of services like Notion, GitHub, telegram etc.
Quite astonishing actually and definitely one of my favourite pieces of software.