The circumstances of the Firestorm Girls' founding is unspecified, nor is their purpose in combat against Doofenshmirtz. However, they seem to be a highly-trained elite unit, whose known troop is troop 46231.
The 1st Dimension
The Firestorm Girls have a small cameo in the movie. They capture Phineas, Ferb, their 1st Dimension counterparts, and Buford, but they release them under the command of Isabella. They (all except for Milly and Isabella) are later responsible for the arrest of Doofenshmirtz upon his return to the 2nd Dimension ("Phineas and Ferb The Movie: Across the 2nd Dimension").
The Firestorm Girls uniform has a standard color of a silver shirt, a dark-green beret on their heads, brown straps worn over their left shoulder that appear to store rolled-up strips of paper, brown pants (with the exception of Gretchen), and black shoes. However, identical to the Fireside Girls members' clothing, every one of the girls' uniforms has a personal touch. The girls look similar to when they were in the video Isabella had when Candace asked her for any unusual video. ("Phineas' Birthday Clip-O-Rama!")
Isabella's uniform is a silver shirt with torn sleeves, black pants with diamond-shaped patches on the portion that covers the kneecaps, black shoes, gray gloves, her beret has a light-green bow on it, and, unlike the other Firestorm Girls, she has a badge with a green circle at the center, with a red lightning-bolt insignia crossing it (probably the Firestorm Girls symbol).
Gretchen's uniform has light-green pants instead of brown, and said pants are tucked in.