The first run of Thunderbird checks if ~HOME/.icedove directory exist, and:
- it should symlink from .icedove to .thunderbird
- or should copy .icedove to .thunderbird - if copy -> you should delete .icedove and .icedove-something* (don't remember) dirs
But, if any problem, I suggest to rename .icedove to .thunderbird dir manually, and remove .icedove* dir as well if any.