I thought that was called "Agile Development." What's unprofessional about it? I'd say it's a lot more professional than letting your browser stagnate for years, leaving security holes unpatched, etc.

As for "how browser development should happen," I think the people who can make the argument on that are the ones who've been leading the pack for the last 5+ years, not the ones who are still trying to catch up despite starting out with a giant lead.