with firefox 3.6.16 a contributor has often this error :

[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: "http://ametysbo.univ-angers.fr/plugins/tiny_mce/resources/js/tiny_mce_src.js Line: 5548"]