The removal of Xwayland breaks firefox build with the error:
/usr/local/bin/clang++90 -o Unified_cpp_toolkit_xre0.o -c  -I/usr/ports/www/firefox/work/.build/dist/stl_wrappers -I/usr/ports/www/firefox/work/.build/dist/system_wrappers -include /usr/ports/www/firefox/work/firefox-71.0/config/gcc_hidden.h -DNDEBUG=1 -DTRIMMED=1 -DTELEMETRY_PING_FORMAT_VERSION=4 -DPROXY_PRINTING=1 -DOS_POSIX=1 -DOS_FREEBSD=1 -DOS_BSD=1 -DUSE_GLX_TEST '-DMOZ_APP_NAME="firefox"' '-DMOZ_APP_BASENAME="Firefox"' '-DMOZ_APP_DISPLAYNAME="Firefox"' '-DMOZ_APP_VENDOR="Mozilla"' '-DMOZ_APP_VERSION="71.0"' '-DOS_TARGET="FreeBSD"' '-DMOZ_WIDGET_TOOLKIT="gtk"' '-DTARGET_OS_ABI="FreeBSD_x86_64-gcc3"' -DGRE_MILESTONE=71.0 -DMOZ_APP_VERSION_DISPLAY=71.0 -DAPP_VERSION=71.0 '-DAPP_ID={ec8030f7-c20a-464f-9b0e-13a3a9e97384}' -DMOZ_BUILD_APP_IS_BROWSER -DTOPOBJDIR=/usr/ports/www/firefox/work/.build -DSTATIC_EXPORTABLE_JS_API -DMOZ_HAS_MOZGLUE -DMOZILLA_INTERNAL_API -DIMPL_LIBXUL -I/usr/ports/www/firefox/work/firefox-71.0/toolkit/xre -I/usr/ports/www/firefox/work/.build/toolkit/xre -I/usr/ports/www/firefox/work/firefox-71.0/toolkit/components/remote -I/usr/ports/www/firefox/work/firefox-71.0/toolkit/components/printingui -I/usr/ports/www/firefox/work/.build/ipc/ipdl/_ipdlheaders -I/usr/ports/www/firefox/work/firefox-71.0/ipc/chromium/src -I/usr/ports/www/firefox/work/firefox-71.0/ipc/glue -I/usr/ports/www/firefox/work/firefox-71.0/other-licenses/nsis/Contrib/CityHash/cityhash -I/usr/ports/www/firefox/work/firefox-71.0/toolkit/components/find -I/usr/ports/www/firefox/work/firefox-71.0/toolkit/components/printingui/ipc -I/usr/ports/www/firefox/work/firefox-71.0/toolkit/components/windowwatcher -I/usr/ports/www/firefox/work/firefox-71.0/toolkit/mozapps/update/common -I/usr/ports/www/firefox/work/firefox-71.0/toolkit/profile -I/usr/ports/www/firefox/work/firefox-71.0/config -I/usr/ports/www/firefox/work/firefox-71.0/dom/base -I/usr/ports/www/firefox/work/firefox-71.0/dom/commandhandler -I/usr/ports/www/firefox/work/firefox-71.0/dom/ipc -I/usr/ports/www/firefox/work/firefox-71.0/dom/webbrowserpersist -I/usr/ports/www/firefox/work/firefox-71.0/testing/gtest/mozilla -I/usr/ports/www/firefox/work/firefox-71.0/toolkit/crashreporter -I/usr/ports/www/firefox/work/firefox-71.0/xpcom/build -I/usr/ports/www/firefox/work/.build/dist/include -I/usr/local/include/nspr -I/usr/local/include/nss -I/usr/local/include/nss/nss -I/usr/ports/www/firefox/work/.build/dist/include/nss -I/usr/local/include -I/usr/local/include -I/usr/local/include/pixman-1 -fPIC -DMOZILLA_CLIENT -include /usr/ports/www/firefox/work/.build/mozilla-config.h -Qunused-arguments -DLIBICONV_PLUG -isystem /usr/local/include -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fstack-protector-strong -Qunused-arguments -Wall -Wbitfield-enum-conversion -Wempty-body -Wignored-qualifiers -Woverloaded-virtual -Wpointer-arith -Wshadow-field-in-constructor-modified -Wsign-compare -Wtype-limits -Wunreachable-code -Wunreachable-code-return -Wwrite-strings -Wno-invalid-offsetof -Wclass-varargs -Wfloat-overflow-conversion -Wfloat-zero-conversion -Wloop-analysis -Wc++1z-compat -Wc++2a-compat -Wcomma -Wimplicit-fallthrough -Wstring-conversion -Wtautological-overlap-compare -Wtautological-unsigned-enum-zero-compare -Wtautological-unsigned-zero-compare -Wno-error=tautological-type-limit-compare -Wno-inline-new-delete -Wno-error=deprecated-declarations -Wno-error=array-bounds -Wno-error=backend-plugin -Wno-error=return-std-move -Wno-error=atomic-alignment -Wformat -Wformat-security -Wno-gnu-zero-variadic-macro-arguments -Wno-unknown-warning-option -Wno-return-type-c-linkage -fno-sized-deallocation -fno-aligned-new -O2 -pipe -O3 -DLIBICONV_PLUG -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -DLIBICONV_PLUG -isystem /usr/local/include -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fstack-protector-strong -fno-exceptions -fno-strict-aliasing -fno-rtti -ffunction-sections -fdata-sections -fno-exceptions -fno-math-errno -pipe -I/usr/local/include -O2 -O3 -fno-omit-frame-pointer -funwind-tables -I/usr/ports/www/firefox/work/firefox-71.0/widget/gtk/compat-gtk3 -I/usr/local/include/gtk-3.0 -I/usr/local/include/pango-1.0 -I/usr/local/include/glib-2.0 -I/usr/local/lib/glib-2.0/include -I/usr/local/include -I/usr/local/include/fribidi -I/usr/local/include/cairo -I/usr/local/include/pixman-1 -I/usr/local/include/freetype2 -I/usr/local/include/libpng16 -I/usr/local/include/libdrm -I/usr/local/include/harfbuzz -I/usr/local/include/gdk-pixbuf-2.0 -I/usr/local/include/gio-unix-2.0/ -I/usr/local/include/atk-1.0 -I/usr/local/include/at-spi2-atk/2.0 -I/usr/local/include/at-spi-2.0 -I/usr/local/include/dbus-1.0 -I/usr/local/lib/dbus-1.0/include -I/usr/local/include/gtk-3.0/unix-print -D_THREAD_SAFE -pthread -I/usr/local/include/dbus-1.0 -I/usr/local/lib/dbus-1.0/include -I/usr/local/include/dbus-1.0 -I/usr/local/lib/dbus-1.0/include -I/usr/local/include/glib-2.0 -I/usr/local/lib/glib-2.0/include -I/usr/local/include -I/usr/local/include/pango-1.0 -I/usr/local/include/glib-2.0 -I/usr/local/lib/glib-2.0/include -I/usr/local/include -I/usr/local/include/fribidi -I/usr/local/include/freetype2 -I/usr/local/include/libpng16 -I/usr/local/include/harfbuzz -I/usr/local/include/cairo -I/usr/local/include/pixman-1 -I/usr/local/include/libdrm -D_THREAD_SAFE -pthread -Wno-error=shadow  -MD -MP -MF .deps/Unified_cpp_toolkit_xre0.o.pp   Unified_cpp_toolkit_xre0.cpp
In file included from Unified_cpp_toolkit_xre0.cpp:56:
/usr/ports/www/firefox/work/firefox-71.0/toolkit/xre/glxtest.cpp:39:10: fatal error: 
      'mozilla/widget/mozwayland.h' file not found
#include "mozilla/widget/mozwayland.h"

I find widget/gtk/mozwayland/mozwayland.h, but no mozilla/widget/mozwayland.h.