About the extension:
This addon does nothing more than give you an editor for the firefox 3.* permissions.sqlite file with import/export capability. When you to go tools > options > content and set what sites you want to allow images on, it places a line in the permissions.sqlite file. Same thing with the cookie preferences, and even the list of websites on which you want to allow installation of addons.
As it turns out, this sqlite file can be used to control 12 other permission types in the exact same way. A single permission just one line in the file, with three space-separated values, naming the domain, the attribute you want to set, and an integer of 0, 1, or 2.
Exexceptions just brings it all together in the same easy-to-grasp window. The file itself is in hex, so in fact exexceptions is an xml (xul?)-based hex editor.
You can also use this extension as a powerful way to control where you can go, what sites are allowed to do, and even change tab behavior a bit. You'll find that the 15 attributes you can control match the names of the html filter types in adblock plus exactly. They are derived from the permissions file.
Go to Addons.mozilla.org and find "exexceptions." -- the version compatibility tag is out of date, so you'll need to go into about:config and set extensions.checkCompatibility to "false" to get it to install. Or if that doesn't work, install "nightly tester tools," restart firefox, then install "exexceptions."
For a basic example of what this extension can lightly be used for, here is some advice I just gave to a user who was asking about the pesky redirection from/to cpalead:
Aerik wrote:If you want to be able to click an ad or watch a video that would take you to cpalead.com if a script is blocked, but not visit that site, create a value for cpalead.com and mark the "document" attribute as "deny". click "set" and you're done. You will never be able to actually 'visit' cpalead.com, nor will you be able to use "save link as" for a url matching cpalead.com. You should also do the same for cpalock.com and adscendmedia.com, which are synonymous with cpalead. Also set the attribute "popup" to "deny." Even if the site manages to hit you with a popup, it'll be a blank, inactive, harmless window you can close.