Mint-X: Unavailable context menu options are not greyed out

[wesinator]

LM 18.2 Cinnamon and MATE with default Mint-X controls

To reproduce:
Open Firefox
Right click in address bar

All context menu options are displayed as regular text even though not all of them are clickable.

Expected: Unavailable context menu items should have greyed out text.

Changing controls to another theme such as Mint-Y immediately fixes the context menu.

[xenopeek]

I can confirm this observation on Linux Mint 18.3 Cinnamon with Mint-X controls.

However I note that this looks to only affect Firefox' address bar context menu. Inactive items in the context menu of LibreOffice Writer, GNOME Terminal and Xed that I also looked at are all greyed out as expected with Mint-X controls. But indeed, switching to Mint-Y "fixes" the issue in Firefox' address bar context menu.