[jamoes]

My interpretation is they're saying that Chromium's address bar code is complex deeply integrated with many other parts of the code-base (including many parts that interact with Google's search APIs). On the other hand, Firefox's address bar code exposes a simple javascript API. This means that making and maintaining custom modifications to the address bar is easier with Firefox.

[bagacrap]

Ever since Chrome Beta users have been able to change the search engine that powers the omnibox (to include autocomplete suggestions, not just search results).

[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.