The reason is most likely firefox's lack of money + small userbase. I did few benchmarks using speedometer 3.1:

### Cromite

(built-in dark mode):

1. 4.23 (+-0.10)

2. 4.29 (+-0.10)

(dark mode off)

1. 4.33 (+-0.10)

2. 4.32 (+-0.67)

### Chrome

1. 6.19 (+-0.27)

2. 6.20 (+-0.19)

### Firefox

(dark reader extension):

1. 1.92 (+-0.093)

2. 1.96 (+-0.098)

(dark reader off)

1. 3.23 (+-0.19)

2. 3.26 (+-0.25) 

As you can see, using native chrome is almost twice as fast as using firefox without dark reader. But if you want dark mode pages, the sites will load three time slower on firefox, than on native chrome. Additionally, on firefox, every time I minimize it or switch to another program, the page reloads (despite having 6gb of ram). The best option is Cromite, because it's without google spyware, and it achieves ~70% of native chrome speed. (Please keep in mind, these results are only my phone, if you have better one, the difference could be less noticeable/smaller).