From MozillaZine Knowledge Base
When a user starts up their browser, this preference is examined. If its value differs from the browser’s current milestone, then the user is redirected to the homepage specified in startup.homepage_override_url and this preference’s value is updated to the current milestone.
Possible values and their effects
The browser’s milestone (e.g., “rv:184.108.40.206”) from the last successful startup of the browser. If it differs from the browser’s actual milestone, it is assumed an upgrade has occurred, and the user is redirected to the homepage override URL. If its value is “ignore”, then the redirect does not occur and the preference is not updated.
First checked in
Has an effect in