Does anyone know how to stop oil frothing out of the breather on a Ford MTX75 box?

Have you checked the oil level? There was a threading running recently about correct depth below filling screw. It seems that filling until it runs out of the hole is incorrect on the Type 9, presumably this will be the same for MT75.

Always filled my previous car's Type 9 till it overflowed but interestingly it was one of Brian Hill's boxes where he added a breather onto the top cover IIRC.