Firefox 12 (currently in nightly channel) has got one essential feature, which was pending for long. The default source viewer of Firefox 12 now has the line numbers to simplify the task of developers.
This feature was pending for almost seven years, and finally got implemented last week. To test it, download (or upgrade to) the latest nightly build of Firefox and launch source viewer for any web page by hitting Ctrl + U.
Firefox is already getting lots of other developer tools. A new “Web Developer” sub-menu has been introduced within the “Tools” menu, and many new features will be added to it with the future releases.