Also happening for me with a reinstall from FC devel from today.
mozilla is ok. no plugins.

Downgrading to the previous version without the language packs in
fc4 works for me.

*** Bug 146604 has been marked as a duplicate of this bug. ***

JFI, firefox-1.0-8 works fine on my rawhide system (rawhide as of
today, as far as the broken deps allow it)

FWIW, I ran into this problem shortly after I installed rawhide, about
two weeks ago, so I went back to FC3's firefox.  Today I figured I'd
try to debug it a bit, so I upgraded to rawhide's firefox, along with
the rest of 20050207 rawhide, started it and, voila, no problem. 
Could it be that it was already fixed elsewhere?  Or is it some form
of race condition and I'd better should keep the old firefox handy?

Still doesn't work on x86_64.

No the bug is on fresh installs.  If you upgrade from a previous
version of Firefox, it works.  On a first time install it freezes.

How do you qualify first-time install?  I did an x86_64 everything
install of rawhide about two weeks ago.  It didn't work.  I removed
firefox.x86_64 and installed firefox.i386.  It didn't work.  I went
back to FC3's firefox.i386.  It works.  Yesterday, I updated to
rawhide's firefox.i386, and it works.  If it fails only for first-time
installs, how come when I installed firefox.i386 it failed?  Are you
saying that, if I rpm -e firefox and up2date -i firefox again, it will
fail again, even though it's not right after a full install?  I'll try
that later.

Yes I believe if you install the new firefox with out a previous
working firefox on the machine it will not work.  If you install the
FC3 firefox and update to the latest, then it works.

Dan

I did this:
rpm -e firefox
rpm -ihv firefox-0.10.1-1.0PR1.20.x86_64.rpm
rpm -Uhv firefox-1.0-8.x86_64.rpm

... and it still doesn't start.  If I use the i386 rpms it does the
same thing.  But, the i386 rpm does work ok on a 32-bit kernel.  Could
it be an x86-64 specific kernel bug (threading, IPC) we're hitting?  I
am using a vanilla 2.6.11-rc2 kernel.

strace shows it stuck in a loop:
write(7, "8", 1)                        = 1
futex(0x604820, FUTEX_WAKE, 1)          = 1
futex(0x2aaaaff0204c, FUTEX_WAKE, 1)    = 1
futex(0x2aaaaff02048, FUTEX_WAKE, 1)    = 1
read(4, "\372", 1)                      = 1
ioctl(3, FIONREAD, [0])                 = 0
poll([{fd=3, events=POLLIN}, {fd=8, events=POLLIN}, {fd=12,
events=POLLIN|POLLPRI}, {fd=14, events=POLLIN|POLLPRI}, {fd=15,
events=POLLIN|POLLPRI}, {fd=16, events=POLLIN|POLLPRI}, {fd=17,
events=POLLIN}, {fd=4, events=POLLIN, revents=POLLIN}], 8, -1) = 1

Same here (except that I was playing with the i386 builds of firefox,
and I was using the latest update as the FC3 package).

However, after running commands similar to yours, I ran:

rpm -U --oldpackage firefox*FC3*.i386.rpm
rpm -U firefox-1.0-8.i386.rpm

and then it worked.

However, comparing the files listed in rpm -ql firefox before and
after this pair of upgrades, no change is there (I tarred them up).

So it must be something in the %post/%pre(un)?install scripts that is
getting it wrong.

Isn't the issue that firefox FC4 is failing if you do not have a previous
firefox profile?

No, it seems to be something systemwide. Having a previous profile copied from a
working system doesn't help.

*** Bug 149504 has been marked as a duplicate of this bug. ***

FWIW, I rebuilt the rpm with the new langpack-installing part commented out of
the specfile, and after installing the resulting rpm, it runs fine.

I commented out the langpack fu for now in rawhide  1.0.1-1 should have this
fix.  Does it work again?

1.0.1-1 works for me (x86-64)

1.0.1-1 ppc is ok after a new login

# grep firefox /var/log/yum.log
Feb 28 08:49:37 Updated: firefox.ppc 1.0.1-1
Feb 28 08:49:43 Updated: firefox.ppc 1.0.1-1

Ok.  Marking RAWHIDE.  Now to figure out why the langpack stuff isn't working =(

The langpacks work in RHEL 4, so why not in FC3 or devel? 

Not sure.  File a bug to re-add them though.