One way this can happen is if you have multiple windows across different monitors or workspaces and you don't close all of them, so Firefox is still running somewhere. This can be avoided by quitting from the menu or with Ctrl+Shift+Q. It may help to increase the number of windows you can undo with '''browser.sessionstore.max_windows_undo''' in [https://support.mozilla.org/en-US/kb/about-config-editor-firefox about:config].