Battle of Varna
It's not certain when the Ottomans started using firearms, however it's argued that they had been using cannon since the Battles of Kosovo (1389) and Nukap (1396) and most certainly by the 1420s. Some argue that field guns only entered service shortly after the Battle of Varna (1444) and more certainly used in the Second Battle of Kosovo (1448). The arquebus reached them around 1425.