For some time now the Debian distribution has shipped a re-branded version of the Mozilla Thunderbird e-mail application called Icedove. The Icedove package contained no trademarks of the Mozilla organization and could be patched or modified as Debian's developers wished. However, times change and Debian is planning to return to packaging the Thunderbird application and will be phasing out Icedove. Christoph Goehre writes: "Hi Debian Developers and followers. Thunderbird is back in Debian! We also renamed other related packages to use official names, e.g. iceowl-extension -> lightning. For now, we need testers to catch existing issues and things we haven't seen
until now. What happens the first time you start Thunderbird? With the change to the official Mozilla branding the users profile(s) will also be changing from '$HOME/.icedove' to '$HOME/.thunderbird' so we need to migrate the profile folder." Details on the transition and things which still need to be tested can be found in this mailing list post.