2.  FirePHP is a Firefox extension that allows you to log data in
firebug. It has a variety of useful logging features, such as
the ability to change your error and exception handling on
the fly and to log errors directly to the Firebug console.
FirePHP

3. PHP Developer Toolbar
 Stop searching at php.net to simply remember some
function signature. With this addon you can search
functions quickly for the official function description
directly into your browser without a new tab.
Plus, many tools to help PHP developers.

4. cssUpdater cssUpdater listen to your FireBug
changes, and live sync your CSS
files.
cssUpdater workflow:
-edit your CSS in FireBug CSS tool
until satisfied
- hit Sync button in FireBug
(hotkey alt s)
- reload your site to see that your
changes are persistent!
- Smile! 

5. Dragonfly
Part of the Opera browser, Dragonfly is
a debugging environment sporting the
usual suspects; it enables you to debug
JavaScript, explore the DOM, preview
images and other resources, investigate
local storage, cookies and more, all in an
interface familiar to anyone who’s used
Firebug

6. FireShot
 is an in-browser
tool for Firefox
and Internet
Explorer that
allows you to
take screenshots
and then
annotate, edit,
organize and
export them.

7. ColorZilla
Is an incredibly
simple, but very useful
extension for Firefox.
If you’ve ever wanted to
determine what colors
are used on a Web
page, ColorZilla is the
tool for the job.
It adds an eyedropper
icon to the bottom-left
corner of Firefox.