The "one actual" firing was the final firing of all 7 Halos by the IsoDidact to wipe out all life in the galaxy in order to stop the Flood. We know of at least 5 Halo firings. > Installation 07 fired over Charum Hakkor, destroying the Precursor constructs there and releasing he Timeless One from its prison. > Installation 07 fired over Janur Qom, the San'Shyuum (Prophet) homeworld on the order of the Master Builder to quell a rebellion. > Omega Halo, last of the 30,000km diameter Halos, fired at Path Kethona to slow the approach of the Star Roads into the galaxy. > The Halo firing itself, wiping out life in the galaxy. > Installation 04B in Halo 3, the ring was unfinished which is why it fell apart. Halos do not generally self-destruct when fired.