I see patches for the problem on https://github.com/mono/gluezilla/commits/master/ from 2011-03-08, not sure if there were any releases - site seems to be down at the moment.
+ 27 Mar 2011; Pacho Ramos <pacho@gentoo.org> -gluezilla-2.6.ebuild, + gluezilla-2.6-r1.ebuild, -files/gluezilla-2.6-fix-xulrunner-detection.patch, + +files/gluezilla-2.6-xulrunner2.patch, -gluezilla-9999.ebuild: + Fix compilation against xulrunner-2.0 (bug #360791 by Rafał Mużyło), + remove old. +