The developers of the Thunderbird project presented a new logo, which was prepared by designer Jon Hicks, who previously designed the Firefox and Mozilla logos. The new logo is designed in the same style as the Firefox logo and emphasizes the connection with Mozilla projects. The new logo is scheduled to ship with the July 115th release of Thunderbird 4. In addition to the logo, Thunderbird 115 will have a significantly redesigned user interface and a new sidebar design mode that is more understandable for novice users (the ability to return the old panel will be preserved).
Source: opennet.ru