Firpo has scored three goals in 16 games this season, including one in the historic 4-3 win over Barcelona at the Nou Camp.
The 22-year-old full-back has been outstanding this season and made his Spain Under-21 debut against Northern Ireland in September.
And rival bosses Roy Hodgson and Rafa Benitez both had scouts in attendance on Sunday, when the 22-year-old hit the only goal in a 1-0 victory over Real Sociedad.
However, Palace and the Toon face stiff competition from Premier League big boys Arsenal and Manchester City for the exciting left-back's signature.
Firpo has become the highest-scoring defender in La Liga in 2018, with five from open play since making his debut in February.
Born in the Dominican Republic, Firpo moved to Spain aged six.
He moved to Betis in 2015 from lowly Puerto Malagueno agreed a new contract at the Benito Villamarin stadium until 2023 in August with a £45million release clause.
And despite the speculation about his future, Firpo is adamant he is focusing on his football.
He said: "I've heard about interest from other clubs for certain. I've told my agents not to tell me anything during the season.
"Nothing can be done until the transfer window opens.
“But ideally I will hear something on January 1."
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