Let Firefox automatically accept "authentication required" dialogue
Do note that, accepting all HTTP authentication dialogs automatically can be risky, because then you don't know when you have sent your credentials and when you didn't.
Good news is that there is a plugin for that: AutoAuth.
AutoAuth automatically submits HTTP authentication dialogs when you’ve chosen to have the browser save your login information. (If you’ve already told the browser what your username and password are, and you’ve told it to remember that username and password, why not just have it automatically submit it instead of asking you each time?)