The good news is that I can tell you exactly where to find the files to edit: in the "chrome" subdirectory of your Firefox profile directory (that icon's part of the Firefox theme) will be all the info you need.

The bad news is that you're dealing with the Mozilla theme system, which hasn't changed since the Elder Days. Here's how I'd go about changing that icon:

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1. Open up chrome.rdf, scroll through until you find the entry for the theme you're using.
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2. Go to wherever that theme is on the filesystem. It'll be a JAR file. Copy it somewhere where you can work on it.
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3. Open up your working copy of the JAR, go into the "global" directory and you'll see a directory called "icons".
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4. Look through those for the tab-close icon, which will probably be called "close.png". Change it to whatever you'd like.
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5. Pack up the JAR file.
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6. Switch to a different theme in Firefox, and uninstall the theme you're using.
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7. Restart Firefox.
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8. Reinstall the theme from the JAR you made of your edited copy.
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9. Restart Firefox.
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I've not actually done this in a very long time, though, so it might not work.