Firefox Split View is now on by default in the latest Firefox 149 Nightly builds so you can add an existing tab to a split view or multi-select two tabs for the split viewing mode.
I tried out the latest Firefox Nightly Linux build and indeed the split-view mode is working as advertised.
The Firefox Nightly builds now having split view mode enabled by default was mentioned in today's Firefox Nightly News .
That Mozilla blog post also noted the addition of the new browser.aiwindow.enabled configuration option to open a new AI Window. That Firefox AI Window is still in its "extremely early days". The AI chat functionality also requires being signed into a Mozilla Account.