user.jsに以下を記述

//reset startup counter every startup
user_pref("browser.slowStartup.samples",0);
user_pref("browser.startup.homepage_override.mstone","");
//tab 1 on startup
user_pref("startup.homepage_welcome_url","https://www.yahoo.co.jp/";);
//tab 2 on startup if any
user_pref("startup.homepage_welcome_url.additional",",http://ja.wikipedia.org/";);