Changing the search engine that powers the omnibox is different than changing the behaviour of the omnibox, which is kind of a lot when it comes to predictive search, predictive completion and a bucket of other things that I wish you could turn off (or can be turned off for a while before Google decides you shouldn't ever need or want to turn it off). It's a target with a huge amount of churn. I agree that Firefox is a more sane target as a developer.