my firefox have same question, sometimem not often.
About Firefox: version: Mozilla Firefox for Ubuntu canonical - 1.0 Mozilla/5.0 (X11; U; Linux i686; zh-CN; rv:1.9.0.8) Gecko/2009033100 Ubuntu/9.04 (jaunty) Firefox/3.0.8
my firefox have same question, sometimem not often.
About Firefox:
version: Mozilla Firefox for Ubuntu canonical - 1.0
Mozilla/5.0 (X11; U; Linux i686; zh-CN; rv:1.9.0.8) Gecko/2009033100 Ubuntu/9.04 (jaunty) Firefox/3.0.8