I used Chrome and Samsung one and Firefox. There is no perceptible difference in speed of the browser, only in speed of mobile internet. I switched to FF a year ago as a main mobile browser and it works just fine.
No difference that maybe you can perceive, but I for one find the scrolling a lot more sluggish on FF than on Chrome, and that's on a SD 845 device, which is the latest and supposedly greatest Qualcomm chip. I just can't use FF on Android, and that's a shame because the desktop version is very good.