Startup.homepage welcome url
From MozillaZine Knowledge Base
When Firefox is started with a new profile (usually, the first time itâ€™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