Modifying patch-br to check for < 299000000 as below did the trick,
Firefox now works reliably when doing DNS lookups.  That answers my
general question I had out about this on current-users and port-i386 too.

> 
> From: David H. Gutteridge <dhgutteridge@sympatico.ca>
> Date: 2005/09/07 Wed PM 08:55:23 EST
> To: <grant@netbsd.org>, <pkg-manager@netbsd.org>, <gnats-admin@netbsd.org>
> CC: <pkgsrc-bugs@netbsd.org>
> Subject: Re: pkg/31170: firefox package build dumps core
> 
> I found patch-br which specifically references this issue.  When
> I originally looked at it (after I grepped pkgsrc for the version
> issue in general last week), my brain was a bit fried and I misread
> the "<" as a ">" (since ">" was what was typically coded for in
> those version checks).  So I thought it was turning on behaviour
> rather than turning it off.  (I remember thinking, "why LOCK_DNS
> if the functions are re-entrant?" but it didn't occur to me to 
> think about it for a minute and realize I was looking at it
> backwards.)
> 
> So I've modified the patch to look for < 299000000 and am
> recompiling it now, will report back when I'm done, I'm sure this
> is the answer to the problem.
> 
> (I could smack myself for not really looking at the code.)
> 
> > 
> > From: grant beattie <grant@NetBSD.org>
> > Date: 2005/09/07 Wed PM 08:29:52 EST
> > To: "David H. Gutteridge" <dhgutteridge@sympatico.ca>
> > CC: tls@netbsd.org,  pkg-manager@netbsd.org,  gnats-admin@netbsd.org, 
> > 	pkgsrc-bugs@netbsd.org
> > Subject: Re: pkg/31170: firefox package build dumps core
> > 
> > On Wed, Sep 07, 2005 at 02:36:45PM -0400, David H. Gutteridge wrote:
> > 
> > > Hello fellow Firefox build sufferer,
> > > 
> > > I've successfully built Firefox several times on 2.1RC3/i386, several times
> > > because if you do get it running, you may find it has what appear to be
> > > problems relating to netdb functions.  I've put some emails out about the
> > > issue, but don't know of a solution at this point.  (See for instance
> > > http://mail-index.netbsd.org/current-users/2005/09/05/0003.html)
> > > 
> > > Basically, DNS queries hang, and they can do so even when the browser just
> > > starts up and calls the home page that's set.  This appears to be related
> > > to the fact that the pertinent function(s) in netdb.h are not thread-safe,
> > > but I'm a bit confused as to why the exact same package built with the very
> > > same options under 2.0.2/i386 works without such problems, I haven't figured
> > > that out.  (So I may be following a red herring.)
> > > 
> > > I have two Firefox builds in package form, the plain vanilla version and one
> > > with debugging symbols and output enabled.  If you're willing to trust some
> > > guy you don't know on the internet (ha ha), I can make them available to you.
> > 
> > "ugh".
> > 
> > I think this is a fallout from the __NetBSD_Version__ change in 2.1,
> > because firefox/mozilla/thunderbird test __NetBSD_Version__ and use
> > thread-safe resolver functions if it's recent enough.
> > 
> > my guess is that the change in NetBSD 2.1 means this code is turned on
> > in Firefox, but I think 2.1 lacks the thread-safe resolver functions,
> > so it breaks.
> > 
> > look at patches/* and see if you can tweak the version number to do
> > the right thing..
> > 
> > grant.
> > 
>