Usually if you restart Windows, Firefox is not running at startup, so you definitely should not get that message. You can check the Windows Task Scheduler and disable the Mozilla tasks there (for example, the one that checks for updates and the one that checks your default browser). Maybe one of those is getting stuck in memory? You also could check the Windows Task Manager (Ctrl+Shift+Esc), Processes tab, to see whether any instances of firefox.exe are running and right-click > End Task/End Process on them. Finally, there is a file named parent.lock that might be locked in a way that Firefox believes indicates that your default profile is in use, and therefore conclude that Firefox is running. I'm not sure how to release that lock if restarting didn't do it automatically. ([[Profiles - Where Firefox stores your bookmarks, passwords and other user data]])