Rated 3 out of 5 stars
If your a developer your better off with WebDeveloper Tool Bar as it offers more featuresThis review is for a previous version of the add-on (22.214.171.124-signed).
It's ok to use both :)
There are times I need the items offered in Web Developer Toolbar, but usually it's turned off to save the space of an entire toolbar. In any case, to each their own, the extensibility of FireFox is part of what makes it so great.