I finally got around to installing the Web Developer add-on. Last time I checked, which was a long time ago, it wasn’t yet updated for Firefox 3. Today I was browsing through the extensions site and realized that it has been updated.
If you are a web site designer and/or developer, you might want to have a look at the Web Developer add-on. It adds a toolbar that gives you a bunch of web development related functions. One that I find useful, is the ability to highlight elements, like DIVs. I use that to figure out where my (table-less) layouts go wrong.
On the downside, it adds a huge toolbar. I wish there was a key-binding to quickly hide/unhide it (maybe there is one, but I’m not aware of it).