Firefox Error: Firefox already running

Firefox Error: Firefox already running

I recently got Firefox on Linux. Most of the time it works like a charm. But once in a while it spews out an error saying something to the effect: "Firefox is already running. Close already running browser first...", even though Firefox is not running. The problem persists even after a reboot.

Based on my experience with VNC, I was looking for a lock file in home directory and /tmp. I removed some things but that did not help either. Does anyone know if this issue and how it can be resolved?

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