MesureIt
Allows to to draw a ruler-marked box over any page to get dimensions in pixels. Great for seeing what size a replacement image needs to be, etc etc.

MeasureIt Firefox Extension(external link)

Search Status
Shows the current Google PageRank and Alexa Rank for the page in the status bar. Also provides a variety of Webmasterly tools such as link checker, keywords density analyser, backward links etc etc.

Search Status Firefox Extension(external link)

Spiderzilla
Spiderzilla is for website-mirroring - it spiders and downloads a copy of a website to a designated folder on your PC. Useful for saving the contents of a site before you upgrade it. It uses the command line program HTTrack.

Spiderzilla Firefox Extension(external link)

ColorZilla
ColorZilla is an eye dropper, colour picker and more. Easily get the hex or rgb ref for any colour on the page by using the ColorZilla dropper. Invaluable when sorting out colour schemes.

ColorZilla Firefox Extension(external link)

Firebug
Possibly the most-used of all the extensions, firebug can be opened in a new windows or bar at the bottom of the browser. It allows you to inspect your HTML, CSS and Javascript code, nicely showing the properties of each element. It offers many useful features such as file-load timers, Javascript debuggers and element outlining.

If you find yourself regularly editing and debugging website code, Firebug is indispensable:

Firebug Firefox Extension(external link)

Web Developer Toolbar
The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools helping you work quickly with cookies, CSS files, Forms, Images and page resolutions. Quickly disable CSS, outline all block elements, show passwords in form fields, display line guides on a page, these are all easily and quickly achievable.

Web Developer Toolbar(external link)