If you don't compile SeaMonkey on Windows or are not using the latest changesets from Bills http://www.wg9s.com site you can skip the rest and stop reading now
I added support for Microsoft VS2019 for 2.49.5 and 2.53 today. Thses two will no longer compile with VS2015. We really didn't test VS2015 in the last year and even the official SeaMonkey 2.49.5 was done with the VS2017 compiler (Yes it is real and just needs to be put up right now). 2.53 also has Windows clang 8 support added (same as current Firefox).
If you want to compile 2.49.5 with VS2019 you need to install the VS2017 141 toolset because of the needed Windows XP compatibility.