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.