I don't know how many of you use Firefox, I use it almost exclusively. Anyway, I hadn't bothered to look at the extensions in awhile, as Venkman (the JS debugger) always crashed and I didn't use it much.

Here are some great developer tools for all the FF users:

Firebug - probably the most popular dev tool. I can't work without it now. Helps track down all those pesky markup and JS bugs.

Execute JS - opens a window where you can edit multi-line JS code and either execute it in the browser or in it's own window.

Console2 - haven't used it much, but it's a great improvement over the default JS console.

JS view - great tool that allows you to view all included Javascript and CSS files on any given page. No more hunting down source code to find the included files!

SQLite manager - for those of us that use SQLite on a regular basis, a really nice GUI tool to manage / create SQLite dbs.

Live HTTP headers (and Header Spy) - two really nice tools to view the actual HTTP header response on any page.

I've got every one of these installed and I can't say they're "bug free" but all seem to be compatible and work well.

The other one I've got is non developer related but I can't do without - Foxmarks. Allows you to synchronize bookmarks across any PC anywhere (as long as it's got an internet connection, and a browser would be pretty useless without one I think). Great for keeping two or three PC's synced with the same bookmarks.

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I reject your reality, and substitute my own.

Awesome list SOCK. I'll be sure to try those, and here's a few more extensions to check out.

Colour Picker - Quickly get the colour of any pixel (saves me screening to paint every time). Also let's you zoom in on pages and measure distances between pixels.

Add N Edit Cookies - Can be very useful in certain situations.

Adsense Notifier - Without this i'd be spending half my day refreshing adsense for stats. Displays statistics in the bottom right corner of firefox.

User Agent Switcher - Adds a menu and a toolbar button to switch the user agent of the browser. Useful when cloaking.