Hi Javier,
I am trying to determine out what might have cause this issue, I hope you can help me to perform a simple test. It should take a couple minutes of your time.
Can you open this file, <install dir>\ccsv5\eclipse\plugins\maqetta.core.client_1.0.0.<buildid>\WebContent\pagedesigner.html. Uncomment the line require.on('error', moduleLoaderErrorHandler), line 143. And remove the mozilla browser property in ccstudio.ini.
See if you can open up GUI Composer with the default browser.
Thank you very much!
Patrick