Jep exactly. All xulrunner and all webkit browsers started to crash when booting -9 instead of -8.This can be time-consuming, but it most likely can give useful information. I would try using svn-bisect against FreeBSD repository to find the regression. Since unpatched kernel is unbuildable on GNU userland, this would require an FreeBSD system with Debian GNU/kFreeBSD chroot.
First please try current stable-8 snapshot. The problematic part might be already backported or due to current toolchain. Petr