One of the most popular (if not THE most popular) Firefox extensions for web developers is Firebug. With Firebug, developers get a powerful web inspector and dev tool that for many (myself included) is now a critical part of their browser and dev experience.
Firebug 1.6 was released this week, providing users with new command line features for power users. On the core HTML side Firebug 1.6 has improved handling for iFrames, (which has always been a sore point for me).
The other interesting addition is support for Firefox’s private browsing mode (aka porn mode) for those times when you want to inspect code on sites that you don’t want tracked in history and/or cookies beyond the session.