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.
The issue is problematic for users who try to restart Firefox on their system. A restart should close the browser process properly before the new process is created. Sometimes, the old process may hang which in turn prevents the new process from starting up.
While you can kill the hung process manually, and most Windows users do probably, inexperienced users may not really know what to do in this case.
Sometimes, waiting may help but there is no guarantee for that.
Starting with Firefox 34, a different message is displayed to users when Firefox is started while the old firefox.exe process is still running.
Users can click on close Firefox to forcefully terminate the process on PCs running Windows Vista and newer, and start a new instance of Firefox afterwards. Why no Windows XP? According to the lead developer, an undocumented API would have to be use for that and the team decided against making that implementation.
The close Firefox button does the same as killing the process in the Windows Task Manager. This means that you end up with a browser that was not closed correctly, which in turn may mean that your last session may not be restored properly if you have configured Firefox to load it.
This is however not different from killing the process manually, as it too will lead to that. Besides, you can also click on cancel to wait a little bit longer before you restart Firefox on the system manually.
The fix addresses a symptom of an underlying issue and not the root cause of it. According to Mozilla, efforts are being made to find the root cause of the issue and resolve it as well to avoid errors like this in first place.
Related to this is another bug that Mozilla is currently working on. The organization plans to shut down Firefox forcefully after a set amount of shutdown time.
This more or less runs the "close Firefox" process automatically so that users won't experience the "is already running issue" on restart.