Another trick worth trying when you're making lots of JavaScript changes like this is to add the following line to your .mozconfig file:

This will cause the .jar files to be expanded so that you can edit the .xml/.js/.xul files in place and skip the repackaging step above (see http://www.mozilla.org/build/jar-packaging.html). If you also use the [http://ted.mielczarek.org/code/mozilla/extensiondev/index.html Extension Developer's extension] you can reload the chrome without restarting the browser.