"A copy of firefox is already open. Only one copy of firefox can be open at a time." What do I do?

I was installing Firefox 4.0.1 and something went wrong. Now every time I try to open Firefox, an error message that says "A copy of firefox is already open. Only one copy of Firefox can be open at a time." I've looked it up and every forum says something about a terminal.. and lock files.. Could somebody PLEASE break it down and explain it as if you were talking to a 2 year old? I am computer illiterate.

Maybe Firefox messages have been modified slightly. Please read the article
*[[Firefox is already running but is not responding]] <-- clickable link -- <br> (the illustrated articles will explain a lot better than I could)
The article explains how to find and stop the extra Firefox process that is causing problems. (You usually use Windows Task Manager to do this - if it is a Windows computer) There is a selection option on the help page with a + by it towards the top right, click that to change the help page details to match the operating system and version of Firefox you use.
If the problem keeps occurring it will be because Firefox has some files mixed up, for instance multiple copies of the files it uses to try to get Firefox to display the details of the last browsing session when you restart Firefox. You now need to follow the link from the above article to this one:
* [[Firefox Hangs]]

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    See:
    * https://support.mozilla.com/kb/Firefox+is+already+running+but+is+not+responding

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    See "Hang at exit":
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    *https://support.mozilla.com/kb/Firefox+hangs

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    See:
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    ** and also [[Firefox hangs or is not responding - How to fix]] <br />& http://kb.mozillazine.org/Firefox_hangs#Multiple_sessionstore.js_files

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    See http://kb.mozillazine.org/Profile_in_use

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    See [http://kb.mozillazine.org/Profile_in_use Profile in use - MozillaZine Knowledge Base]

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