This page is a Stub
The discussions on this newsgroup helped me realize that another way of explaining the user interface in 1.4 may help people familiar with the 1.3 mechanism.
In 1.4 you add a domain to the whitelist by opening Firebug on a page in that domain. So for example if you open the URL http://getfirebug.com, then open Firebug, the domain getfirebug.com is added to the whitelist. If you subsequently open http://blog.getfirebug.com, Firebug will be opened because getfirebug.com is on the whitelist.
In 1.4 you add a domain to the blacklist by clicking [X] in the upper right corner of Firebug when Firebug is open on a page in that domain. Continuing the example above, while on http://blog.getfirebug.com, if you click the [X] button Firebug closes. When you go back to http://getfirebug.com, Firebug is still closed because getfirebug.com is on the blacklist.
A few details:
First the mechanism used for the tests is closer to the same-origin policy of Firefox than simple domain comparison. The case that will probably confuse people is http vs https: Firebug like Firefox considers http://getfirebug.com to be different than https://getfirebug.com.
Second, this description assumes you have 1.4b5 or later and that you leave the default option Activate Same Origin checked.