well, you still have an outdated language pack (10.0.1) installed. Close Firefox, open Synaptic, reload, and install the firefox-es_ES-10.0.2, then restart Firefox, you should be asked if you want to have the new language installed, and your answer should be yes, then restart FF and it is in Spanish, or go into the FF-Languages tab and enable the new 10.0.2.
-p.