For web development:

• Firebug – All round awesome web development tool. Allows for on-the-fly alterations to CSS, the HTML DOM. Inserting break points into JavaScript, viewing any existing JSON objects in memory and providing a JavaScript interpreter for debugging.
• YSlow – Useful web profiler that adds itself to Firebug. Provides recommendations on how to optimise web pages as well.
• Web Developer Toolbar – I prefer Firebug overall but Web Developer has a few extra features like resizing the window for different screen resolutions.
• NoScript – Easy approach to enabling and disabling separate JavaScript files on a website.

My favourites:

• Compact Menu – Allows you to get rid of the regular ‘File’ menu in Firefox and provides either a small ‘Globe’ icon or the simply the word ‘Menu’ that’ll drop down and provide the regular Firefox menu when clicked. Saves a lot of screen real estate for the user.
• Fission – Turns the address bar into a loading indicator by representing how far loaded the page as a coloured bar spreading across the address. Similar to Safari’s address bar loading system.
• Download Status Bar – Does away with the regular ‘Download Window’ of yesteryear and provides information on the status of downloads on the regular Firefox window via a pop-up status bar.

An essential (but detrimental to this blog’s revenue!) add-on:

• AdBlock – Automatically hides (doesn’t even waste bandwidth on) adverts according to automagically maintained universal regular expressions.

Note: The ordering of the add-ons is not meant to indicate my preference or importance I place on them