Before you come at me with stuff like Librewolf, Waterfox and IceCat; those don’t count. They are just tweaked Firefox distros with mostly basic low level changes. Not every Chromium browser is super unique either, but I feel like there are more differences between them then there are with Firefox distros. Why is that? Why there aren’t different browsers that use Firefox’s engines but provide a different UX?
Above would be the primary reason. From a business strategy perspective website compatibility is better with Chromium than Firefox. While Firefox is fast now, there was a period in the mid 2010’s when Firefox was trying to diversify its income streams that resulted in it taking it’s eye off the ball and the browser experience got slower and suffered. We’re still seeing this effect even though they have improved the browser significantly over the past couple years.