I've been playing around with splitView for a few days now, and it's in an excellent state for a brand new feature, but there are some friction points that could do with some quality of life massages to really shine as a browser feature.
When you first open SplitView, you're presented with a list of tabs from that window which you can immediately add to that side of the view. I frequently have many tabs wit very similar names, and regularly click the wrong link, and the mistake is cumbersome to correct.
It isn't possible to fully solve the 'click the wrong tab' problem, but rendering a preview-box of the tab when you hover over the link would allieviate the issue for tabs wit similar names but visual distinctive content. Whether Firefox does so could be a togglable accessability or performance option.
Alternatively, the selection menu could simply be rendered as a grid of previews rather than a list of tab names. This alternative would also want to be implimented as togglable under accessability or performant options.
(I definitely typed and posted my QoL suggestion in the correct order. The server just seemed to eat suggestion number 3 for whatever reason, so here it is again, I guess.)