Right-click on disabled layer in layer control on firefox brings up browser context menu, not layer context menu

[prushforth]

Steps to reproduce:

In firefox, visit: https://maps4html.org/experiments/custom-projections/BNG/

• hover over the layer control, the second layer Surficial Geology - British Geological Survey is disabled
• right click over the disabled layer in the layer control

Expected behaviour:

• the layer context menu, containing "Zoom to layer" and "Copy Extent" buttons is displayed

Actual behaviour:

• the browser context menu is displayed

Might be a Leaflet issue.

[Malvoz]

Another problem with disabled items in the layer control is that keyboard-only users cannot access them (if that's important).