Firefox in Ubuntu : how to automate basic authentication password confirm dialog

"Try xdotool or IronAHK. I haven't used either of them, but they're basically tools for simulating mouse and keyboard events, which sounds like one way of solving your problem.

Edit: See the comments for the exact answer."

"There is an automation addon for firefox called iMacros.
I have not used it first hand and cannot vouch it it will help you solve your problem.
You could give it a short try."