Startup.homepage welcome url
From MozillaZine Knowledge Base
When Firefox is started with a new profile (usually, the first time it’s started after installation), the URL specified in this preference will be loaded (even if the user imported their own homepage). This is similar to startup.homepage_override_url, which specifies the URL loaded when Firefox is first started after an upgrade, but this preference allows specifying the page a first-time user would see.
Possible values and their effects
The URL to display upon loading Firefox with a new profile for the first time. (Default value is http://%LOCALE%.www.mozilla.com/%LOCALE%/%APP%/%VERSION%/firstrun/ for officially-branded builds; http://www.mozilla.org/projects/%APP%/%VERSION%/firstrun/ otherwise)
First checked in
Has an effect in