I like their choice in going with Opera.
First of all, PSP went with Mozilla, so having Opera on the DS brings us into a cool new portable browsing war(yay?).
Secondly, although FF might be more stable, faster, or whatever else, Opera has much more innovation, or atleast has the characteristics of a company with it, which suit's the DS's overall outlook on it's fanbase. Anyone actually seen the DS on the internet? from a console with two screens, I think they did the best they could possible, and it suited the DS's overall style in terms of it's UI and even the body of the DS perfectly.
Lets all remember, it's not like PSP or DS is using 1:1 replicas of the browsers provided to them, they're simply asking these companies for help to create usable browsers on their system. I would never compare PSP's browser to FireFox or Mozilla, and I think the same applies to the DS's browser and Opera.