Firefox extension that shows the site's IP address in the browser's address bar. Just hover your mouse over the icon to display it or click to copy.
You can install it from addons.mozilla.org.
For faster IP display on hover open about:config and add the preference ui.tooltipDelay with the type Number and the value 100 (default is 500).
To adapt the icon color to the theme color open about:config, find option svg.context-properties.content.enabled and change it to true. See bug #1377302.