In recent years, Mozilla has handled two main projects: Firefox and Thunderbird. That’s about to change though, according to Mozilla CEO Mitchell Baker on her blog. The reason, of course, is so that the most important focus can be shifted towards Firefox only. So, where is Thunderbird going? No one knows yet, although options are flying around. TB will still of course use the same Mozilla base it always has had, it will just be under a different direction. Thunderbird is my preferred e-mail client, so whatever happens, I hope development continues to go strong.
While the Mozilla Foundation supports a number of projects, its taxable subsidiary the Mozilla Corporation is responsible for only Firefox and Thunderbird. However, it has become increasingly clear that Firefox is the priority. The resources allocated to Firefox dwarf those allocated to Thunderbird and recent projects such as the initiative to improve Mozilla support exclude Thunderbird.
Source: MozillaZine