Yes for both thunderbird and firefox, ever since they started 
  adding /usr/pkg/lib to the LD_LIBRARY_PATH they set 
  in /usr/pkg/bin/{thunderbird,firefox}.  I've been hacking that entry 
  out post install on NetBSD with no ill effect.

  It was added to fix an issue on Solaris - see rev 1.22 of 
  Makefile-thunderbird.common and rev 1.2 of firefox/INSTALL.  The odd 
  thing is I thought I'd tried conditionally not adding it for NetBSD 
  but it had still ended up in the script and I hadn't tracked down 
  why - hence no patch from me.

Do you mean that setting LD_LIBRARY_PATH is a pkgsrc-specific change?
If so, we really should stop, or should clean the environment before
shelling out.  Should I be withdrawing my thunderbird bug report?