There did not seem to be a pattern to it though. The only indicator was the Windows Task Manager and a Firefox process that was still running although the browser had been closed before. Waiting was no option either, the process did not close after waiting multiple times for 15-30 minutes.

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.

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?

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