From Muppet Wiki

Ernie's window box, located outside the window of Ernie and Bert's apartment, is a small garden which also serves as the home of the Twiddlebugs. The Twiddlebug family resided in a milk carton home, decorated with miniature furnishings, such as a postage stamp of George Washington. In the more recent computer-generated appearance, the home has been updated to a Chinese food carton; however the inside is the same (complete with the upside-down Washington stamp on the wall).

Other Twiddlebug homes and communities exist in other window boxes on Sesame Street. Twiddlebug Town is located in Elmo's window box.