I used to be pretty ontop of the latest plugins but it's lost my attention recently (probably due to so many itentical blog articles called 'the best {num here} addons for firefox').
Last time i looked i found:
firequery and firepicker which are now indespensible,
so what's new?
any hot new extensions that have found their way into your daily work?
The first one from a recent answer, design-oriented:
Firebug, accessibilty and help to keyboard implementations: Firefocus will track the element that currently has focus
QuickJava allow to easily turn on or off Java, Javascript, Flash, Silverlight and Automatic loading of images from the Statusbar and/or the Toolbar without having to open the Options or Plugins dialog!
Must have!
Performance related: the HTTP Archive (HAR) format is being adopted by numerous tools. This format allows you to compare a website performance day after day or 1 year later and brings interoperability.
Not new, but one that I use nearly constantly and few others I tell about it seem to know of: Web Developer 1.18 (http://chrispederick.com/) Originally, I found myself using it most for highlighting divs quickly and easily. Later, on one particularly difficult job I realized that I can use it to reliably toggle CSS, Javascript, etc with a simple checkbox. Also, a tool called "view generated source" has allowed me to easily identify issues that popup in IE and Safari that firefox is intelligent enough to know about and fix on its own (what a concept, Microsoft) Used in conjunction with Firebug, there's almost no UI issue I can think of that should stop you.
Google Chrome has a page of featured developer extensions. And since the Google Chrome extension system is pretty new, all these extensions are pretty new.
I personally use a few of them on a regular basis. Besides the good ones already mentioned, I’d say Speed Tracer is extremely handy.