docx is Microsoft Office 2007 Word documents extension. Now, you can view docx files using Firefox by installing OpenXMLViewer, which is available for Windows, Mac and Linux users.
The picture above shows the architecture of OpenXML Viewer. After installing the Firefox plugin, you are able to open any docx files using Firefox.
It is an useful extension for users who haven’t upgrade to Microsoft Office 2007. Basically, the extension will convert docx documents into HTML format so that web browsers can render and display to users. So, the extension is not able to shows embedded objects.
Source: Digital Inspiration