This is how they are adding features to the browser. This is a smart way of doing it. Make it experimental, gauge the feedback, and integrate it if the feedback makes it seem useful.
As an example, they tried vertical tabs, and it appears it didn't work as well as they needed it to be so it has been left out. On the flip side, the empty tab page was also an experiment, and that worked well and made it to the default browser.
These experiments are work on the browser. But instead of adding them as features, introducing them as experiments and then adding the polished, stable, feedback adjusted version as a true feature that can be marketed effectively.
Vertical tabs used to work so well on Firefox that I held on for just that, even when everybody had already left for Chrome. Then they killed it. Now you're saying they dropped it because they couldn't get it to work again. Kind of sad.
As an example, they tried vertical tabs, and it appears it didn't work as well as they needed it to be so it has been left out. On the flip side, the empty tab page was also an experiment, and that worked well and made it to the default browser.
These experiments are work on the browser. But instead of adding them as features, introducing them as experiments and then adding the polished, stable, feedback adjusted version as a true feature that can be marketed effectively.