[CentOS] Firefox, Thunderbird crashing X Centos 5.11

Wed Oct 15 15:04:17 UTC 2014
Negative <negativebinomial at gmail.com>

I cannot start Firefox or Thunderbird ever since I tried to put in a new
graphics card.

I had trouble with the configuration on this machine, which has two
monitors. Once the new card was in I decided to change the configuration
from cloned to spanning screens.  I messed up and didn't save the old
working configuration. Along the way, I wondered if the new card didn't
have a problem and I put the old one back. At some time in all this,
 firefox began crashing on startup, but that wasn't the primary problem.

The old card, now installed, is an  nvidia Quadro fx580.  I use El-Repo's
kmod-nvidia and driver. It's up to date.

The X error appended to the Xorg log on the last FF crash Is thus:

Backtrace:
0: X(xf86SigHandler+0x71) [0x4868d1]
1: /lib64/libc.so.6 [0x39c4c30030]

Fatal server error:
Caught signal 11. Server aborting

(II) Screen 0 shares mem & io resources
(II) Screen 1 shares mem & io resources

Any thoughts, suggestions?

Thanks