That’s all I know, (not much) more information in the Project-Blog.
today I switched from Build 20190329130005 to latest available Server Installation of unofficial (by wg9s) en-US SeaMonkey 2.53 (NT 6.1; Win64; x64; rv:56.0) Gecko/20100101 Firefox/56.0 Build 20190122130014 (Default Classic Theme, German Language Pack active) on German WIN7 64bit. And the first thing I saw was that message again. Strange! AFAIR I only see that on my Browser Home Page, but I am not sure.
I will ask at mozilla.support.seamonkey.