> When contextual suggestions are enabled, Mozilla receives your search queries. When you see or click on a Firefox Suggest result, Mozilla collects and sends your search queries and the result you click on to our partners through a Mozilla-owned proxy service. The data we share with partners does not include personally identifying information and is only shared when you see or click on a suggestion.

I wish they'd give the about:config key to disable it as well as UI instructions in posts like this. The former can be set in a preferences file; so easily synchronised between machines and not forgotten when setting up a new one.