How to convert pixel relative focal lengths to mm.
There is only one focal length, $F$. It is the distance between the focal point and the image plane. 

The numbers you have, are actually not lengths. They are calculated by $f_x=F\cdot s_x $ and $f_y=F\cdot s_y $  where $s_x$ and $s_y$  are the size of your image in pixels. To get the focal length in mm as you want, simply divide $f_x$ with the width (in pixels) of your image.