Some users have reported that the Clippings submenu on the browser context menu would often disappear when moving the mouse to select a clipping or folder on the menu. There have even been reports of crashes occurring when this happens.
This is caused by a bug in the Video DownloadHelper extension which causes problems with many context menus in Firefox. If you have this extension installed, upgrade it to the latest version (4.0.2 at the time of this post). This will resolve the issue.