Comment 2 for bug 222327

I can verify this bug. I marked my own reported bug (#234874) as a duplicate:

Description:
Minimizing Firefox 3.0b5 may crash X-server if the Visual Effects settings in Gnome (Under System -> Preferences ->Appearence -> Visual Effects) are set to "Normal". Also, I use the nVidia-driver.

The following may reproduce what I believe is the same bug:
Start a new session of Firefox and open youtube.com. Open a new tab and choose that tab. Start clicking on the Firefox-tab in the Gnome Panel/taskbar so that Firefox minimizes and restores itself. Click quite fast (approx 2-4 clicks per second) and sometimes up to 50 or more clicks in total. Eventually the X-server will crash and the login-screen appears.

The following is from /var/log/syslog at the time of the crasches:
"
May 25 01:59:12 ubuntu-desktop gdm[6110]: WARNING: gdm_slave_xioerror_handler: Fatal X error - Restarting :0
May 25 01:59:17 ubuntu-desktop kernel: [39584.743570] agpgart: Found an AGP 3.5 compliant device at 0000:00:00.0.
May 25 01:59:17 ubuntu-desktop kernel: [39584.743773] agpgart: Device is in legacy mode, falling back to 2.x
May 25 01:59:17 ubuntu-desktop kernel: [39584.743780] agpgart: Putting AGP V2 device at 0000:00:00.0 into 2x mode
May 25 01:59:17 ubuntu-desktop kernel: [39584.743826] agpgart: Putting AGP V2 device at 0000:01:00.0 into 2x mode
May 25 01:59:34 ubuntu-desktop pulseaudio[8031]: pid.c: Stale PID file, overwriting.
May 25 01:59:38 ubuntu-desktop hcid[5228]: Default passkey agent (:1.119, /org/bluez/passkey) registered
May 25 01:59:38 ubuntu-desktop hcid[5228]: Default authorization agent (:1.119, /org/bluez/auth) registered
May 25 01:59:46 ubuntu-desktop NetworkManager: <info> Updating allowed wireless network lists.
May 25 01:59:46 ubuntu-desktop NetworkManager: <WARN> nm_dbus_get_networks_cb(): error received: org.freedesktop.NetworkManagerInfo.NoNetworks - There are no wireless networks stored..
"

I have been unable to reproduce this bug without opening the second tab in Firefox or if the Visual effects are set to "None".

Other than this bug, the system is stable. Viewing flash-content in Firefox works fine.