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I am running Windows 7 and just yesterday the following message was displayed and I can't log into Firefox:

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  • Siste svar av dynamite

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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.

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.

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This problem is caused when Firefox process does not exit even when you close the application. See if the solutions given in this article help: Firefox Hangs.