Layer control disabled layers contextmenu doesn't work on firefox

[prushforth]

1. Layer control disabled layers contextmenu doesn't work on firefox

This may be because of an underlying bug in firefox, but it may be possible to work around, with some effort, because the default context menu is shown.

2. Context menu always appears at center of map on firefox, making "Copy Location" unlikely (but Paste also doesn't work on firefox, so perhaps that's a moot point until both are fixed).

This is also likely because the contextmenu event is not a kind of PointerEvent on firefox, so the coordinates of the right-click event aren't reported.

There are known upstream accessibility issues with context menu.