This implements the "clip" attribute value for the "crop" attribute found in XUL label elements. It functions similarly to having the text-overflow property set to an empty string or "". This shouldn't break previously established behavior as it only adds a new value to be checked.
The behavior of the "none" attribute value in the documentation (https://udn.realityripple.com/docs/Archive/Mozilla/XUL/Attribute/crop) is incorrect. The "none" attribute value literally means "don't crop anything" if you'd check the code and is also equivalent to not setting the "crop" attribute at all. This has always been the case since Firefox 2 (earliest version I checked) and apparently, this section of the documentation was copied verbatim from XULPlanet without even checking if it's correct.