Nothing worked - until I checked the exact version Fellipe has posted!
So in order to make the ugly xulrunner message go away (I am using Windows 7 x64, Eclipse Kepler 4.3.2 x64 and Java 8 x64) and even be able to use something else than DefaultType = IE, you should really attempt to use the 10.0.10 version of XulRunner.
For me it was relatively easy to figure out if it’d work or not: I just double-clicked “xulrunner.exe” and checked whether the version information was readable. Other versions than 10.0.10 were unreadable, but when I installed 10.0.10 and did that, it properly displayed the version number and Eclipse instantly discovered and used it (and so did the Visual Designer of Vaadin 7).
Thank you very much!