Mercury glass Christmas ornaments that are currently being produced do not contain mercury. This unfortunate name has negative connotations that are not deserved. These ornaments most likely contain silver, not mercury, if made today.
Mercury would not be acceptable for use in Christmas ornaments due to its toxicity. Even encapsulated there would be the danger of breakage, so mercury would not be used. This is the same kind of misnomer as the red liquid in thermometers that often is referred to as mercury. Indeed, the liquid in thermometers is often alcohol colored with red coloring or mineral spirits.