To preview HTML file stored in JCR you don't have to open additional browser window. With eXo IDE all you need is to click on "Show Preview" Toolbar button or select "Run->Show Preview" from top menu after opening HTML file in Content Panel. After this HTML-content will be opened as Web-page in IFRAME of Preview Tab.
It is impossible to view just created and non-saved files.
Unsaved changes are not displayed when you edit HTML file.