I love to use that preinstalled microb. I like it has flash plugin, it is quite fast, it has good switching between windows.
Firefox is very very slow, and is without flash support.
I didn't tried Opera yet.
Chromium is good, it also has possibility of flash but is much slower even when overclocked than default browser. Plus I don't like it has no zooming possibility and overall is quite from another world, I think it doesn't fit good in maemo.