I'm seeing perhaps half a dozen hits per day on the lines of:
Mozilla/5.0 (Windows NT 6.1; rv:6.0) Gecko/20110814 Firefox/6.0 Google favicon
Access seems always to be home page.
The IP range claims to be proxies but is also used for a few utils such as translation, feedfetcher, wireless transcode etc.
It's getting refused mainly because Firefox 6 is obsolete and the UA mentioned favicon. Header fields in general are reasonable though not entirely what an average user might supply.
I cannot find a legit visitor counterpart to these accesses in the logs. They seem to be isolated events.