more annoyingly, this issue can only be diagnosed by running Browser/start-tor-browser --verbose. ./start-tor-browser.desktop and Browser/start-tor-browser simply output nothing.
I downloaded stock Firefox ESR 52.8.0 and ran it with -P non_default_profile -no-remote and it started with no issues. it comes with both "libmozgtk.so" and "gtk2/libmozgtk.so" which I assume helps.