It used to happen to me every time on RH9 and FC6, using Firefox 1.5 (and I believe also 2.0), after the browser was used for a day or so.

Now, with FC6, FF 3.0.10 it doesn't happen very often, but it happens, especially recently.

** (evince:16737): WARNING **: Unimplemented named action: POPPLER_DEST_FITBH, please post a bug report in Evince bugzilla (http://bugzilla.gnome.org) with a testcase.

** (evince:16737): WARNING **: Unimplemented named action: POPPLER_DEST_FITBH, please post a bug report in Evince bugzilla (http://bugzilla.gnome.org) with a testcase.

** (evince:16737): WARNING **: Unimplemented named action: POPPLER_DEST_FITBH, please post a bug report in Evince bugzilla (http://bugzilla.gnome.org) with a testcase.

** (evince:16737): WARNING **: Unimplemented named action: POPPLER_DEST_FITBH, please post a bug report in Evince bugzilla (http://bugzilla.gnome.org) with a testcase.

(Gecko:28775): Gtk-CRITICAL **: gtk_drag_set_icon_pixbuf: assertion `GDK_IS_DRAG_CONTEXT (context)' failed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x28d6e54 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x28d6e47 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x28d664f unexpectedly destroyed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_user_data: assertion `window != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_back_pixmap: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_x11_visual_get_xvisual: assertion `visual != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_hide: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_user_data: assertion `window != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: _gdk_window_destroy_hierarchy: assertion `window != NULL' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x28d6650 unexpectedly destroyed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_user_data: assertion `window != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_back_pixmap: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_x11_visual_get_xvisual: assertion `visual != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_hide: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_user_data: assertion `window != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: _gdk_window_destroy_hierarchy: assertion `window != NULL' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x28d6653 unexpectedly destroyed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_user_data: assertion `window != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_back_pixmap: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_x11_visual_get_xvisual: assertion `visual != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_hide: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_user_data: assertion `window != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: _gdk_window_destroy_hierarchy: assertion `window != NULL' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x28d6664 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x28d6637 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x28d6672 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287af57 unexpectedly destroyed

(Gecko:28775): Gdk-CRITICAL **: gdk_x11_visual_get_xvisual: assertion `visual != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_user_data: assertion `window != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_back_pixmap: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_move_resize: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_hide: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_show_unraised: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287af5a unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287af1f unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287aeef unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287afc4 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287afb5 unexpectedly destroyed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_get_origin: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ae6f unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ad2c unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ad6b unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ad59 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ad58 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ad43 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287afcb unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287abf7 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287af28 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287af09 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287afaa unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287af86 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ae0d unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287afb6 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ad81 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ae48 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ad88 unexpectedly destroyed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_user_data: assertion `window != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_back_pixmap: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_move_resize: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_hide: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_user_data: assertion `window != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_back_pixmap: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_set_data: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_move_resize: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_hide: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_show_unraised: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_show_unraised: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_hide: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_user_data: assertion `window != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: _gdk_window_destroy_hierarchy: assertion `window != NULL' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_hide: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_user_data: assertion `window != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: _gdk_window_destroy_hierarchy: assertion `window != NULL' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287afae unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ad2e unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ad2d unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ada6 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287af00 unexpectedly destroyed

(Gecko:28775): Gdk-WARNING **: GdkWindow 0x287ad30 unexpectedly destroyed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_hide: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_hide: assertion `GDK_IS_WINDOW (window)' failed

(Gecko:28775): Gdk-CRITICAL **: gdk_window_set_user_data: assertion `window != NULL' failed

(Gecko:28775): Gdk-CRITICAL **: _gdk_window_destroy_hierarchy: assertion `window != NULL' failed

(Gecko:28775): GLib-GObject-CRITICAL **: g_object_unref: assertion `G_IS_OBJECT (object)' failed