Do you have the Firefox icon on your quick launch? Right click on the icon and select Open New Window. Sometimes this works. If it opens, close it by Menu > File > Exit.

This is not a cure but will make it easier if Firefox locks up in Windows. https://support.mozilla.org/questions/997866 {web link}

Firefox may not be closing cleanly, leaving the firefox.exe process running.

Open the Windows Task Manager (right-click in the Windows Task Bar, click Start Task Manager) and then open the Processes Tab. Hi-light firefox.exe and then use End Process.