Google stopped innovating the "Browser" for legacy versions of their operating system. Just like Microsoft. However, the Marketshare of legacy Android is still huge, all stuck with the old "Browser".
https://developer.android.com/about/dashboards/
ARMv6 builds of Firefox are coming soon:
http://armenzg.blogspot.com/2012/06/initial-automated-armv6-...
I would have liked to see them do the same for the browser app a long time ago, but in a way having Chrome as a separate app achieves the same goal going forward if it becomes the default.
1. http://gs.statcounter.com/#mobile_browser-ww-monthly-201105-...