app.update.interval (Integer). The value entered here specifies the amount of time, in seconds, that Firefox waits before checking for updates. The default being 86400, aka 1 day. Of late the main Firefox releases have been due to security issues and as such it would be wise to leave this at the default to ensure you get notified of such updates ASAP.
Basta abrir a página: config na barra de endereço, digite app.update.interval
e altere o valor de acordo.