i tried uninstalling and re-intalling firefox , both in greek and english language,but the result was the same.i manage to download videos from  youtube , but after restarting my computer ,  the add-on doesn`t work.the same happens whith x-mail notifier.i used these add-ons for at least 2 years without any problems , but suddently they dont work.
  at first i thought it has somethink to do with adblock plus,but i also used it for 2 years with no problems at all.
   please help me

although i downloaded download helper,the add-on doesn`t work when i restart mozilla (seems inactive,asks for reactivacion and restarting of firefox but nothing happens) . i tried uninstalling and re-intalling firefox , both in greek and english language,but the result was the same.i manage to download videos from youtube , but after restarting my computer , the add-on doesn`t work.the same happens whith x-mail notifier.i used these add-ons for at least 2 years without any problems , but suddently they dont work. at first i thought it has somethink to do with adblock plus,but i also used it for 2 years with no problems at all. please help me