Private Browsing only applies to bare Firefox. How extensions and plugins behave in Private Browsing mode is not defined, so it is quite possible that a plugin leaves traces elsewhere. For MS plugins this would include the IE temporary Internet folder and the IE cookies storage. Other plugins like QuickTime and Java may use their own cache. Firefox needs to pass files to a plugin and probably saves them in the temp folder as usual when a file is passed to an external program.

I think you didn't really absorb what I said . Why does every one make the generalized assumptions and provide stock answers without trying to apply the data to the symptoms described. Did you try what I described and see whether you observed the same response? Of course not, because you know different, so why bother (or maybe you are just lazy)

Try this and get back. Use windows 7 64 bit and firefox 21

1. Open normal window 2. open private window (there are now 2 windows) 3. Open avi file from the internet from the private window. Observe its creation in the temp directory (needs to be from internet to show up in temp (not temp internet files)). I have my firefox set to open avi files in an external viewer. If it plays it in firefox I'm not sure it will create a file in the temp directory. 4. stop playing the avi file. Notice it still exists (normal) 5. Exit the private window and notice the file still exists (not good). 6. Exit the non-private window and notice the file goes away 7. Rationalize why you think this is normal behavior since Firefox can do no wrong.

It's a bug --- a not-completely-implemented-feature-BUG

Am I in the wrong forum? No answer? Seems simple enough.