The song "The Donkey Serenade" was not in the original stage production. It was written for the film, but the music was adapted from the song "Chansonette," from the stage version of "The Firefly."
Except for the songs, this movie has absolutely nothing whatsoever to do with the operetta, "The Firefly."
Jeanette MacDonald married Gene Raymond on June 16, 1937, just after her scenes in the picture were completed. They were together for the rest of their lives, 28 years, until her death in 1965 of heart disease.
The original release prints featured color tinting of sequences in Sepia-Blue, Sepia-Orange, or Sepia-Blue-Pink. Unfortunately, only plain black-and-white re-issue prints have been used for VHS and DVD releases and for TV broadcasts.