Openning Xml file in firefox

I'm beginner in XML, and this is the first time I create a XML file, so excuse me if it's not a good question

My XML file sometimes opent in firefox sometimes not!

When it contains:

<?xml version="1.0" encoding="utf-8" ?>
<boxName>name1
    <boxColour>colour1</boxColour>
    <boxFrom>from1</boxFrom>
</boxName>

It opens correctly in firefox but when it contains:

<?xml version="1.0" encoding="utf-8" ?>
<boxName>name1
    <boxColour>colour1</boxColour>
    <boxFrom>from1</boxFrom>
</boxName>
<boxName>name2
    <boxColour>colour2</boxColour>
    <boxWeight>weight2</boxWeight>
</boxName>
<boxName>name3
    <boxColour>colour3</boxColour>
    <boxFrom>from3</boxFrom>
    <boxWeight>weight3</boxWeight>
</boxName>

It doesn't open!!!

Do you know what is the problem?