I have a problem with sub-menus. I have a Navbar with three sub-levels which have worked fine for a couple of years.

Now I have just noticed that when hovering the mouse over any of the sub-menus, after one second the menu disappears.
This makes it impossible to navigate to the required page, and has only happened in the past week.

I have WD 11 Premium 64 bit v11.2.5.42127

The issue seems to be restricted to Firefox.
In Windows 7 64 bit, I'm running running Firefox 52.0.1 I've tried Firefox in safe mode, but it's the same problem.
Windows Explorer, Opera, and an old PC version of Safari all work fine.

In Linux Mint I have Firefox 52.0 64 bit (mint 1.0). The sub-menus disappear after one second in Firefox here too.
Using Qupzilla in Mint there is no problem, the sub-menus display correctly.