If at first you don't succeed, try again on the big screen. That's what ''Buffy the Vampire Slayer'' creator Joss Whedon decided when Fox canned his 2002 series ''Firefly'' after just 10 weeks. Now all systems are go. Universal's given Whedon a $35 million-plus budget to write and direct ''Serenity,'' starting production this summer. Named for the spaceship in which Firefly's band of renegades troll space 500 years from now, the feature includes cast originals Nathan Fillion (Capt. Malcolm Reynolds) and Gina Torres (Reynolds' partner Zoe). ''I felt as strongly as ever that these characters had a story in them,'' says Whedon, who promises new thrills for fans -- 200,000 of whom bought the series on DVD. ''It's pretty damn cool.''