The newest versions of Firefox use a different default handling tabs in a newly started instance of Firefox - while earlier versions loaded all tabs at once, newer versions only load the active tabs. This is supposed to save loading time, but can be annoying if some of your tabs need an update to indicate new messages, such as twitter, and you forget to load it - the status of the tab won't change at all until you click it once.
If you prefer the old default that loads all tabs at once, you can reset it in the Firefox preferences. To get there,
- open the Firefox menu and select Options > Options. This will open the preferences window.
- On the General tab, select Show my windows and tabs from last time from the When Firefox starts drop down menu under Startup.
- Then uncheck the Don't load tabs until selected checkbox.
- Accept the changes by clicking OK.