xdg-desktop-menu-dummy.menu causes Firefox to crash

[blubee blubeeme]

This issue started happening recently I think after updating to the latest
version of Firefox.

Every so often I'll come back to my machine and Firefox has died, then in
one of my terminals I'll see this message:

kbuildsycoca4 running...
kbuildsycoca4(51442) VFolderMenu::loadDoc: Parse error in
"/usr/home/blubee/.config/menus/applications-merged/xdg-desktop-menu-dummy.menu"
, line  1 , col  1 :  "unexpected end of file"

When I look at the file in question it's just an empty file. I tried
grepping around finding that xdg-desktop-menu file but nothing came up.

Anyone have any idea what it's about or why is it crashing Firefox?
Firefox version: 57.0 (64-bit) from ports tree.