The Web Developer toolbar will become one of your most used plugins.It is its own toolbar that gives many useful tools for analyzing, validating and optimizing web-pages, Disabling or enabling scripting, clearing your cache and cookies and much more.
The great thing about this plugin is that it’s on a few different browsers too, so if you switch between them you will be able to use the same plugin on all.
Firebug is pretty much your bread and butter day to day god send!
If your not using this yet you should be. Firebug allows you to view the HTML and CSS of any website and make live changes to both of these sources. You can see how things look and then copy the code and make changes your actual files. Firebug though is much more then this, You can view a pages live console activity from errors, script interactions, post information, cookies and more.
It comes across with few different powerful options and supports addons of its own such as firecookie to edit and view cookie information dynamically.
SeoQuake will help you optimise your web pages for search engine optimisation (SEO).
It gives you an toolbar that has a number of useful SEO-related options for you to take advantage of. This extension help you in identify issues pertaining to search engine indexing in your web pages.
YSlow is provided by Yahoo, it is a utility addon that benchmarks a web page’s front-end performance. It will assist you in optimizing your web designs for fast response times by identifying performance issues.
(Take some information with a pinch of salt too, location and rendering factors always have to be considered, your never going to get a perfect score but aiming for as best you can is always a great goal to achieve.)