From The Book of Boba Fett – Season 1 Episode 4: ‘Chapter 4: The Gathering Storm’ (1×04) | Produced by Lucasfilm & Disney+
What is a Firespray?
We and all the fans were surprised to hear Boba adress his ship as Firespray, while we all know it better as Slave I.
Slave I (or Slave One) is actually a name of a modified ship model ‘Firespray-31-class patrol and attack craft’, which Jango Fett and then Boba Fett owned. In short: Slave I is a name of a specific ship, while Firespray is a name of the ship model, which was massively produced.
In this case it was better for Boba to use the model of the ship, as the name Slave I would not mean anything to Fennec.