i have problem in firefox when running iis it asks username and password

i fix the problem by changing IIS property to checked Anonymous users

this time IIS home page is running but i try to run .net application its not working. it return authentication error that s its not login for the the user

IIS create default password for authentication username (IUSR_"username")
how to set password for (IUSR_"username") i am trying to set but its not set
iis defaltly set password. if its set pasword how to get that password

how to solve this problem