This page has information about my favourite Firefox plugin - Adblock Plus, which can be installed from Firefox's addons page.
If you want Adblock Plus to allow Firefox to display ads on a particular website, for example, to test the embedded adsense ads in your blog. Adblock Plus allows you to do this by setting up an exception filter in the preferences window. If you want to allow ads to be displayed on http://www.example.com then add the following filter:
@@|http://www.example.com
It should appear in your filter list as an exception rule and Adblock Plus will no longer block ads on http://www.example.com.