Mozilla-tan, a subset of OS-tan, is the personification of the Mozilla Foundation's creations Mozilla Firefox and Mozilla Thunderbird.
Firefox-tan
Aka Firefox-ko. A personification of the browser Mozilla Firefox. Little is known about this character, but she's portrayed as a red foxgirl, in a shinto priestess outfit. Its color set and design resemble that of the Firefox logo. Variants include 九尾狐, an imaginary fox-like creature that has 9 tails.
Versions
Thunderbird-tan
Aka Thunderbird-ko. A personification of the mail and news client Mozilla Thunderbird. She is the best friend of Firefox-tan. She dresses like a telephone operator, carries a keyboard/monitor, and sometimes holds a letter in her hands.