The actor, who plays MI5 boss Harry Pierce, told What's on TV that the BBC spy drama has had "the luxury of longevity".
"It's a process of trial and error making film and television," said Firth. "Over the years... the show's just got better, and that's very rare in television."
He continued: "Frequently something will peak within two or three seasons, and then fall off. The [shows] that have managed the long haul really stand out, particularly in British television."
He added that the makers of Spooks have "learnt from [their] mistakes".
"We can watch where we've got it wrong... and then make sure we don't do that again," he suggested. "Now we're in our tenth year, and if we haven't got it right by now, then something's sadly wrong!"
Firth also admitted that he currently knows nothing about the show's tenth series, which is expected to air on BBC One later this year.
"Scripts usually arrive the day before production is due to star and sometimes not even that [soon]," he claimed. "Sometimes we just get a few pages."
> Q&A: Peter Firth chats Spooks!