Experimental support of 64-bit Windows is available in JBoss Tools 4.1.0.Beta1 and JBoss Developer Studio 7.0.0.Beta1.
To enable it, you need to install XULRunner from this update site: http://download.jboss.org/jbosstools/builds/staging/xulrunner-1.9.2_win64/all/repo/.
Please comment this JIRA if something went wrong.
Many thanks to Carsten Pfeiffer, who ported XULRunner 1.9 to 64-bit Windows and shared the sources with us!