I have the same problem almost one year later.
Newest Version of Chrome works fine. Newest Version of FF (default settings) blocks the new windows with the inbuilt popup blocker.
Also the Construct documentation states
[quote:2huv8mv1]Open URL in new window
Navigate to a given URL in a new window (or tab if the browser settings override). This continues to run the HTML5 game in the old window or tab.
How to change the default way to open the URL to "tab" instead of "window"? I doesn't need to work for customized browser settings, but should at least work for the default settings.