The easiest way I've found to start at user logon the default browser (firefox in my case) and open a default page was to create a simple script to be executed at the user logon through a group policy. The script is really simple, It must be a .bat that launches the command @start followed by a parameter corresponding to the page to open, example:
@start http://www.google.it
Be careful, if the URL contains the characters '&’ It must be rewritten with the escape character '^’ so for example:
@start https://www.google.it/search?num=100&safe=off
becomes:
@start https://www.google.it/search?num=100^&safe=off
The gpo to be created then is the classic for scripts that must start at logon; creating the file that will contain the script directly from the gpo management it will be created in the folder containing the gpo inside the folder 'policies’ in 'sysvol’ so it will be automatically copied to all domain controllers: