Many users have reported that on Firefox 3 with Menu Editor installed, the Clippings submenu on the browser's context menu won't display any clippings and folders. This problem is reportedly reproducible on Firefox 3; this problem is not occurring on Firefox 2.
This conflict occurs once Menu Editor is installed and its extension preferences dialog is invoked from Tools → Add-ons in Firefox.
There is presently no workaround to this bug. Disabling or uninstalling Menu Editor appears to be the only way to bring back your clippings and folders in the Clippings submenu on the browser's context menu.