I do. Deafault browser is slow, the Aurora builds for Firefox have been the fastest option I've found so far on my GNEX.
But if Chrome beta is faster I'll use it. On mobile it's all about speed for me!
I've never seen any chart or graph where Firefox Mobile even shows up. Usually when something has <1% marketshare it gets categorized as statistically insignificant or grouped in "other".
It's still not nobody. And having Firefox playing a significant role will prevent situation which existed in the past where many sites were "IE only". We don't need "WebKit only" sites.
There is a difference between "WebKit only", which means you have to support some N number of standards (CSS 1/2/3 for example) and "IE only" sites, where you have to run Windows and support ActiveX.
There are enough WebKit browsers specific things which are not standard to make "WebKit only" situation a problem. In general, well balanced browsers representation makes web developers think better and be more responsible.
Do you have evidence that they are correctly detecting Firefox Mobile? In my day-to-day use, I've seen it treated as everything from desktop Firefox to Mobile Safari, so I'm not terribly optimistic on that point.