A recent article on the Specyboy blog provides a summary of some of the most useful firefox plugins for web designers.  These include the excellent web developer extension which I have used for several years.  A lot of the other plugins perform tasks already covered by the web developer plugin, however there is the odd interesting one.  These include abduction a screen capture tool and  extended status bar which provides information on load speed.  The full list can be seen here, 27 Indispensable Web Design and Development Firefox Extensions.