If you don't compile SeaMonkey yourself you can just stop reading.
If not, starting today the setup changed with 2.53.6b1 pre. Like 2.57 and recent Thunderbird releases >= 60 the comm sources need to now be in a subdirectory named comm under the mozilla sources. Previously the comm sources where located in the root directory and the mozilla sources in a mozilla directory under it.
Also previously the application was named suite in the mozconfig. This is now comm/suite. Everything else stays the same.
The upcoming 2.53.5 beta 1 and later the final will use the old setup.
FRG