Hi,
Forward your problem to the current-list and tech-pkg lists as well, 
this is may be related to changes in -current. Even if 'firefox' builds 
with your 4.99.40+ userland & kernel , it will dump core on startup. I 
tried with sources 12/08 and the dynamic linker problem is still there. 
The last  -current userland which allows me to run firefox and all 
kde/gnome programs was 10/17/07, i can use a recent kernel 4.99.39 
(12/04) with my 4.99.33 userland. This box:

casey234: {1} uname -a
NetBSD casey234.switchpwr.com 4.99.39 NetBSD 4.99.39 (GENERIC-$Revision: 
1.320 $) #0: Tue Dec  4 12:24:49 EST 2007  
mel@casey234.switchpwr.com:/usr/obj/sys/arch/alpha/compile.alpha/NEW alpha
-164Lx with mail sent from thunderbird, all built from pkgsrc.
Mel