I’m a huge, huge fan of the latest Firefox release. But I was really, really disapointed that a LOT of my very favorite (and most used!) plugins were incompatible. The plugin I felt most lost without was Aardvark, which I use every single day, practically all day long to aid me in development, editing and design.
Aardvark is a simple little Firefox plugin that when activated (right click and choose “Start Aardvark”) lets you highlight each elements on the page with a red border – not only that, a tab appears on the border with the element and id/class that is associated with it.
This plugin is critical for me (along with Firebug) when diagnosing layout problems.