Logout forward doesn't work on Firefox version 23 or later
"Logout Forward" feature is built with iframe (sourcing configured logout URL).
But by default, Firefox (from 23 version), blocks mixed contents (http/https) This is set with configuration parameter "security.mixed_content.block_active_content=true" So, if LL::NG Portal URL is on HTTPS, none logout request is sent for applications where logout URL is defined on HTTP.
Replace "iframe" by "img" prevents changing Firefox configuration (security.mixed_content.block_active_content=false)