Love the idea, been looking for exactly the same solution, but after installing the extention it worked one time and then after restarting my browser it would not let me use my browser even though i am putting in the correct password, i did not even had access to the settings to remove it. i had to remove the extention manually by deleting its XPI file from the firefox folder.

Great idea, but it is supposed to lock others from my browser not lock ME out of my own browser.