[xemdetia]

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.

[bagacrap]

The gp's parenthetical addresses what you call "predictive completion", only it uses the term "autocomplete".

[Dylan16807]

It addresses changing the site that feeds it. It does not address changing the behavior.