The message “Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system” popped up quite frequently on a test system running the latest official version of Firefox 3.
The issue was noticed regularly after closing Firefox and opening the browser soon thereafter again.
There did not seem to be a pattern to it though. The only indicator was that a firefox.exe process was still shown as running in the Windows Task Manager even though all windows of the browser had been closed already and non was visible anymore on the desktop.
First time I noticed the issue I decided to wait to see what would happen. In particular, I was interested if the Firefox process would terminate automatically. Turns out it did not and after waiting 15 or so minutes I decided to troubleshoot the issue to find a solution for it.
The only option that helped was to kill the Firefox process which would free the resources and make it possible to restart Firefox. The problem with killing the process though is that it could damage important files that are needed to run Firefox. It could damage files in the profile folder like the bookmarks.
The Mozilla website lists a few possible causes that could be the root of the problem. The lists includes problematic extensions, Java applets or Internet security software. There is also an error description in the Firefox Knowledge Base about the Firefox is already running error message which gives a few additional pointers.
These pages list many different causes for the issue. A common one is that Firefox is still processing data in the background which can be the case if you have configured it to delete browsing data on exit for example.
This process should not take longer than a couple of seconds tops though.
You can terminate the process but that may cause data loss and is generally not advised unless there is no other option to deal with the issue.
One solution may be to delete the parent.lock file in the Firefox profile folder if it exists as it may be responsible for the issue.
The problem is that non of the possible causes listed on the two pages were the cause for the error message on the test system. It is easy to test if extensions are the cause of the error by starting up Firefox in safe mode which will load Firefox without extensions. If the problem persists it is clear that extensions are not the source.
Anyone else ever got that problem and found a solution for it?