(In reply to Eitan Isaacson [:eeejay] from comment #1)
Jamie, originally I suggested to Morgan that we implement a "Show menu" action in our platform code because I didn't want to mess with the brittle MSAA legacy actions we support in nsIAccessible. Marco mentioned there may be interest in having a context menu action in Windows too?
The problem with actions on Windows (and I'd argue with our current action API across the board) is that there's no differentiation between primary and secondary actions. So, we have to assume that action 0 is primary. Even in the absence of a primary action (e.g. click, press), I'd argue that "show context menu" should not be treated as the primary action. To put this in terms of a user scenario, a user won't expect pressing enter on something (even if it isn't clickable) to open the browser context menu.
In short, I think there's definitely a theoretical benefit to having context menu actions on all platforms, but we'd need to find some way to deal with this primary/secondary action problem. On Windows, there's no current practical benefit because screen readers, not having such an API for years, just route the mouse and right click.