Firebug
Firebug is an amazing web development extension for Firefox. Anyone who does any web development should use it.
I started using it a few versions back, primarily as a replacement for the built-in JavaScript console. Its logging features were very appealing (I tended to use alert() before that, and that just isn’t a lot of fun), and generally I found it rather useful to have around. It was already on my list of must-install plugins.
Now, though, it’s really ridiculous. First of all, it now allows for multi-line JavaScript editing in the browser, something that’s unbelievably useful. It allows for all kinds of CSS and DOM inspection and alteration, live, from the browser. Its JavaScript in-browser editing has a format-into-bookmarklet function (I really wish I’d had that two weeks ago!). It has a “net” tab that shows you, individually and together, the load times and the byte weights for all the components of every page loaded. Its command line has tab completion. It has JavaScript stack traces and assertions.
The only downside so far is that it clashes with one of my other favorite extensions, HTML Validator—if both are enabled, I can’t type anything into form fields on a page without ‘find as you type’ taking over and grabbing all the input. I’m hoping this will be fixed soon, but even if it remains forever, both extensions make it worth the trouble of disabling/enabling and restarting Firefox.
If you do web development, go get Firebug.