This can be the result of using MS plugins like the Windows Media Player or Silverlight unless you would be using an extension like IE View or IE tab that uses the IE rendering engine (that do not show in the System Details List).

Thanks cor-el for the reply. Do not have any MS plugins that I'm aware of. No Silverlight,etc. How can I tell if Windows Media player is plugged in to FF? No IE View or IE tab.

Not a major problem because I can go in and delete manually--minor hassle. Have had the same problem across several FF updates. Just curious why this is happening.

You can try to correlate the time and date of the files in the IE Temporary Internet folder and the IE cookies folder with the Firefox history to see if you can find a match which website might be involved and check those out for plugin usage.

Thanks, will give it a shot.