Some Firefox add-ons like DownloadHelper work in the background and wait to download a video until the video has been completely rendered on your screen, so if you try to close Firefox while such an add-on is working, you will get that message. If you ever installed an add-on like that and then uninstalled it, you may have residual data in your Firefox profile that is telling Firefox there is still an unfinished download. To clear all downloads, from the Firefox menu bar, open Tools -> Clear Recent History -> Time Range Select Everything for a time range, and only check the box below that for Browsing and Download History. This will clear your entire history, so if you have any tabs or windows open that you wish to remember, bookmark those windows and tabs in a temporary bookmark folder before proceeding. Please post back and mark this issue as "Solved" if it solved your issue, and if not please post back with further questions.