Sometimes you get same authentication prompts, for a target. For example, if you are using an HTTP proxy which requires authentication and the home page is cached, then multiple connections for embedded resources are trying to be established and produces multiple authentication prompts for the proxy. This addon simply suppresses such duplicated prompts. If there are multiple prompts with same message, they are automatically filled with authentication information entered into the first one.
Possibly related bugs:
533467 – Prompt proxy password multiple times in one page every page
650091 – Multiple Proxy-Authenticate headers order-dependency bug causes login popup with NTLM + Basic