Bug 508167 will implement support for the NPAPI methods that Adobe Flash will support in Adobe Flash 10.2+, allowing us to tell that application to delete their version of cookies (LSOs) as well.
Previously we have seen a lot of evidence that users are surprised that Flash objects store data on their system and further that this data isn't cleared when browser cookies are cleared. It seems evident that to the user, cookies = cookies, and so the best way to match expectations would be to interpret cookie-clearing actions as meaning "web and flash cookies".
When "Custom" history options are selected in the "Privacy" prefpane, there is an option to "Clear History when Firefox Closes" that has "Settings...", one of which is cookies.
When that checkbox is selected, we should also use the new API from bug 508167 to clear Adobe Flash LSOs. Yes, this will clear all LSOs even those set by other browsers; that's acceptable, IMO.
The implementation of this is identical to bug 625496, since they share the same backend. So we'll get this for free.
Fixed by bug 625496.