When it comes to running Firefox on Windows, the dreaded "Firefox is already running" error message when trying to launch the browser on the system is highly annoying.
This happens under certain circumstances, usually when the Firefox profile is still locked on the system while a new instance of the browser is being started.
This happens for instance if Windows tries to start Firefox while the previous instance of the browser has not been terminated (yet) correctly.
You can check if that is the case by opening the task manager of Windows with Ctrl-Shift-Esc and verifying that a firefox.exe process is still running there.