Bueno, despues de descubrir la forma de controlar los errores, os puedo dar una pista (que a mi no me ha servido) de cual puede ser el problema. El error es este:
Cita: [Exception... "An attempt was made to use an object that is not, or is no longer, usable" code: "11" nsresult: "0x8053000b (NS_ERROR_DOM_INVALID_STATE_ERR)" location: "file:///home/santics/.mozilla/firefox/9nd2g1vu.Santics/extensions/%7Be4a8a97b-f2ad-450b-b12d-ee028ba24781%7D/components/greasemonkey.js Line: 436"]