Historically, Icedove was a re-branding of Thunderbird by the Debian project, with minimal modifications, in order to resolve branding disputes. Debian since discontinued Iceweasel in February of 2017 after reaching an agreement with Mozilla over the use of it's trademark.
The Hyperbola Team continued the Icedove branding in the spirit of cultural freedom for several months. However, inconsistencies were discovered, including the use of non-free fonts in Debian's Icedove logo. Since Debian no longer supported Icedove and was encouraging users to install Thunderbird with the new Quantum Engine, we decided to look into creating a proper fork.
Icedove was the second application to be successfully ported to UXP by Hyperbola Project.