FireBreath's wix code relies on heat, which doesn't work with 64 bit DLLs.
You can make your own .wxs template for it using a 32 bit build and then use it similar to this:
if (CMAKE_SIZEOF_VOID_P EQUAL 8)
# On 64 bit heat.exe doesn't seem to work!
configure_file("${CMAKE_CURRENT_SOURCE_DIR}/Win/WiX/pluginObject.wxs" "${CMAKE_CURRENT_BINARY_DIR}/pluginObject${WIX_HEAT_SUFFIX}.wxs")
message("Configuring ${CMAKE_CURRENT_SOURCE_DIR}/Win/WiX/pluginObject.wxs to ${CMAKE_CURRENT_BINARY_DIR}/pluginObject${WIX_HEAT_SUFFIX}.wxs")
set(WIX_HEAT_OVERRIDE "${CMAKE_CURRENT_BINARY_DIR}/pluginObject${WIX_HEAT_SUFFIX}.wxs")
set(WIX_IS_WIN64 yes)
SET(WIX_PLATFORM x64)
SET(ProgramFilesFolder ProgramFilesFolder64)
ELSE()
set(WIX_IS_WIN64 no)
SET(WIX_PLATFORM x86)
SET(ProgramFilesFolder ProgramFilesFolder)
endif()
add_wix_installer( "${PLUGIN_NAME}"
"${WIX_SOURCE_FILE}"
PluginDLLGroup
"${FB_BIN_DIR}/${PLUGIN_NAME}/${CMAKE_CFG_INTDIR}/"
"${FB_BIN_DIR}/${PLUGIN_NAME}/${CMAKE_CFG_INTDIR}/${FBSTRING_PluginFileName}.dll"
"${PROJECT_NAME}"
)
The main thing to note there are the WIX_HEAT_OVERRIDE which tells it which .wxs file to use instead of running heat. I honestly don't remember what the other things are for, but feel free to check out the code yourself and figure it out :-P That should help you get started, though.