Great tool, with a nasty bug :P Rated 4 out of 5 stars

This tool is a must-have for any FF extension developer.
Unfortunately it manages to break page layout on FF 3.0.

Here is how to fix it.

Go to Firefox Profile directory.

Locate extensions/

Uncompress it (using zip) to a temporary folder.

Edit content/xv.js and comment out or delete the following block:

// ==========
// XXX Only for debugging. Comment this in release version!!!
try {
var _prefs = Components.classes [";1"].
_prefs.setBoolPref("browser.dom.window.dump.enabled", true);
_prefs.setBoolPref("javascript.options.showInConsole", true);
_prefs.setBoolPref("javascript.options.strict", true);
_prefs.setBoolPref("nglayout.debug.enable_xbl_forms", true);
_prefs.setBoolPref("nglayout.debug.disable_xul_cache", true);
_prefs.setBoolPref("nglayout.debug.disable_xul_fastload", true);
} catch(e) {}
// ==========

Re-compress your changes to xpcomviewer.jar

Also You want to remove the preferences set by this code
from Your prefs.js or by using about:config

Good luck!

This review is for a previous version of the add-on (0.9a).