Streep stepped out of her pump while walking up a few steps to accept the prize for her work in "The Iron Lady" Sunday.
Firth, who was on stage to present her with the award, quickly ran to the stairs, snapped up the shoe she left behind and placed it back on her foot, handing the BAFTA mask to her in the process.
"Thank you so much," Streep laughed, giving her former "Mamma Mia!" co-star a kiss. "That couldn't be worse. That couldn't have gone worse. Oh, gosh."
Host Stephen Fry joked about the incident after Streep gave her acceptance speech.
"Prince Charming tried the slipper on and it fitted Cinderella at once. What a happy ending," Fry said.
Firth's "Tinker Tailor Soldier Spy" won the BAFTA for Best British Film.