HTML Validator. If you are an HTML designer, a page developer, or indeed anyone who makes web pages for a living, you need this extension. I can't tell you the number of headaches it has saved me by pointing out exactly where something doesn't match up.

Aardvark. This application is superficially similar to the DOM inspector, but it has a far nicer interface. You turn it on, and all the page elements are tagged for you on the page. This sounds a little abstract, so here's a demonstration:

Firebug. This is a number of different things, and it's already had very good press. It does a number of things, but the best thing from my perspective is that it validates CSS, debugs Javascript (in a way far superior to Venkman) and can track XMLHTTPRequests. If you're working on AJAX applications, Firebug pulls many different things together to form a unified whole.

Scrapbook. This extension changes the way I surf the web. You drag and drop a bookmark or a page into it, and it stores the page for you. You can then read the page offline, without being connected to the web. And because it's local, you never have to worry about losing the URL. I'll drag pages in and use them for reference at meetings, for long articles, for web page receipts...

That's it for now -- there are other firefox extensions I like, but they're personal preferences rather than new features.