A level 1 Fiend-patron warlock finds a spell scroll of fireball. The spell fireball is not on the base warlock spell list. If a spell is not on your list, you cannot even try to cast it from a scroll. That said, it is on the Fiend patron's expanded spell list, which states:
The following spells are added to the warlock spell list for you.
This doesn't appear to be limited by warlock level (unlike a cleric's Domain Spells list), but I may be missing some key statement.
Can a level 1 Fiend Pact warlock cast a scroll of fireball (after making a DC 13 Charisma check)?