Files with the .xul extension are most commonly associated with the Mozilla Firefox Internet browser. The XUL files are XML user interface files. The XUL file format was created to make it easier to develop the Mozilla Firefox browser. Other Web browser that are based on the Gecko engine also use the XUL file format.
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If you have additional information about the XUL file format or software that uses files with the XUL suffix, please do get in touch - we would love hearing from you.