OK...found the problem which appears to be related to gecko mediaplayer.
/var/slim.log shows the following error...
(firefox:11563): GLib-GIO-CRITICAL **: Settings schema 'apps.gecko-mediaplayer.preferences' is not installed
(firefox:11563): GLib-GIO-CRITICAL **: g_settings_get_key_info: assertion `settings->priv->schema != NULL' failed
(firefox:11563): GLib-CRITICAL **: g_variant_get_type: assertion `value != NULL' failed
(firefox:11563): GLib-CRITICAL **: g_variant_type_is_subtype_of: assertion `g_variant_type_check (type)' failed
(firefox:11563): GLib-CRITICAL **: g_variant_get_boolean: assertion `g_variant_is_of_type (value, G_VARIANT_TYPE_BOOLEAN)' failed
(firefox:11563): GLib-CRITICAL **: g_variant_unref: assertion `value != NULL' failed
Removed Gecko Mediaplayer and Firefox appears to be OK now.
Is there anything that I could do to resolve this conflict apart from removing Gecko Mediaplayer?