The hover status becomes false when the mouse pointer is invisible.
To reproduce the bug:

1. Install the unclutter package configured with the "-idle 1 -root"
   extra options.

2. Start Iceweasel.

3. Open https://www.vinc17.net/cijml/index.fr.html

4. Put the mouse pointer over "Menu", then move it downwward over
   the menu.

5. No longer move the mouse pointer.

After 1 second, the mouse pointer becomes invisible (this is what
unclutter does). But the menu disappears (this is a purely CSS menu,
based on hover). This makes some CSS menus nearly impossible to use
because the mouse pointer disappears before one can read the menu
and determine what to do.

This bug is reproducible in safe mode.

Firefox Nightly does not have this bug. So, either it is Iceweasel
specific or it has been fixed upstream.

I've noticed this bug on my new machine because Debian installs
unclutter by default!

The problem disappears after I kill unclutter.

Well, it seems that it wasn't fixed after all. I don't know why
I couldn't reproduce it with Firefox Nightly at that time.