If you are using Firefox 4 on Windows 7, you can easily disable the tab previews by using a simple trick. The trick involves change some Firefox configuration using about:config.
To disable Tab Preview in Windows 7 for Firefox 4 or other Firefox versions, just type in about:config in your address bar and hit enter. On doing that, you might see some warning message saying that "You may void your warranty!", you need to press the button "I’ll be careful, I promise" to move to the configuration for Firefox.
In the Filter textbox, type "taskbar.preview" without the quotes and hit enter. You will see 3 configuration options in the search results. In that right click on the text which says "browser.taskbar.previews.enable" and select the "Toggle" option.
That’s it, once you toggle the value close the about:config page. The settings will be applied without having to restart the browser.