rails Remotipart form with Firefox doesn't work

Question

I have this form, which submits well on Safari and Chrome but not in Firefox :

<%= form_for @user, :remote=> true, :html => { :id => "upload_avatar" , :multipart => true} do |f| %>
<%= hidden_field_tag :delete_avatar_pic%>
<%= f.file_field :avatar, :accept => "image/*"%>
<% end %>

Any idea?

Thanks!

Jeremie