A WMF is a vector graphic format, isn't it? A JPG is a graphic format, isn't it? You're hiding a vector graphic in a graphic. If you're hiding code in that WMF, you're hiding executable code in a WMF, so you've got code in code in code.
Firefox reads metadata for all files, last I checked, and will display them if the proper software is available to decode it.
Example: a GIF is accidentally named .JPG, but firefox will see that it is a GIF and load it.
Example: an EXE is accidentally named JPG, and firefox will not be able to execute this file, so it will ask the user to save it instead. at this point, nearly any image viewer will say "SOMETHING IS WRONG" and it will not be executed as an EXE until then.
What is firefox going to do about a WMF (within a JPG) file that it knows nothing about? it's going to ask the user to save it somewhere.