Interesting. I’ve never noticed it. I’ll try to pay closer attention.
The context menu works fine in version 18.0.2. In subsequent versions, the bug symptoms are: only the bottom and right edges appear when the mouse is right clicked.
In version 19 and up, I can make it happen consistently here on FR by right-clicking between the lines of post. It may be interaction with addons (noscript, ghostery refcontrol).