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