You can bypass the protection. Rated 3 out of 5 stars
I want to see if this will protect against software installing addons on me. However, I opened the add-ons page to modify the settings for them in FireFox 12 and I clicked cancel on the password box to see what would happen. It shows the page in the background and then it goes to a blank page. You can hit the escape key after you click cancel and it won't change the page.
In order to block this hacking attempt, you might consider blocking internal URLS. about:addons is the internal URL for the add-ons page and if you block access to it in a way that will only allow the page to be opened by entering the password, it'll be harder to hack.