Hovering over menu bar drop items roll off when pointer moved
Hovering over the menu bar of some web pages (e.g. https://www.newbluefx.com), opens a drop down list. As I move the pointer towards the list, the drop down disappears and I cannot access the list items.
A similar situation exists when some pages contain a YouTube video insert. In this case hovering over the bottom of the YouTube insert surfaces the play menu. But moving the pointer over the handle the play menu retracts.
This works OK in Chrome.
I am using Firefox 51.0.1, in Windows 10 with all the latest updates.
Is there some setting that I need to change in Firefox, or is this a bug?