Mozilla Thunderbird is a free and open source mail and news client.
Thunderbird-tan is depicted as a fair-skinned woman with short silver hair, bangs and two long locks and golden eyes. She dresses in a black leotard with a futuristic-looking yellow and blue sleeveless split-front dress, baggy, thigh-high stockings, detached sleeves, a white neck-corset, yellow hat and blue headset. Her overall impression is that of a telephone operator and is often shown carrying a keyboard, monitor or letters.
She is depicted as being best friends with Firefox-ko.