The Icedove packages are de-branded back to the official Mozilla branding.
With the removing of the Debian branding the packages are also renamed back to the official names used by Mozilla.
The Thunderbird package is using a different default profile folder, the default profile folder is now ‘$(HOME)/.thunderbird’.
The users profile folder, that was used in Icedove, will get migrated to the new profile folder on the first start, that can take a little bit more time.
Please read README.Debian for getting more information about the changes, and possible needed trouble solving.
Note also #816679 containing some legal facts.