To preview the HTML file stored in the JCR, you do not need to open an additional browser window. With the eXo IDE, you just click Show Preview button on the Toolbar, or go to Run > Show Preview on the top menu after opening the HTML file in the Content Panel. The content of HTML file will be opened as a webpage in the IFRAME of Preview Tab.
You cannot view created and non-saved files only.
Unsaved changes will not be displayed when you edit the HTML file.