To add to the comments not mentioned any UX reasons, I wanted to use Firefox on my phone as I do on my desktop and laptop but when using it I disliked the tab organisation compared to Chrome, and that's pretty much all I need the Web browser to be good for on my phone, where I'll often have up to 100 tabs open. Maybe it's changed now but Firefox had an impractical 3D scroll through tabs showing only the top IIRC, whereas Chrome has a much more practical grid of page previews that allowed for easy navigation. It also now allows one to group tabs very nicely.

I hope Firefox has changed, or that there's a setting hidden away somewhere for these.