I use Yahoo! extension for Firefox, it check for loading times , it rates page's speed by certain parameters.
I also can validate your html code.

Firebug is pretty good....

1. Firebug - needed for debugging javascript quickly, and seeing elements change in realtime via ajax-ish libraries.
2. Web Developer toolbar.
Absolute must. Enable/Disable Images CSS, edit code inline, size of elements.. windows etc.. too much to list here.
3. Selenium for automated testing.
4. Live HTTP Headers - for debugging server POST send/responses
5. Javascript debugger (for times when firebug just can't get in fast enough.

Kind regards, Spencer

Hello All,
I found 11 Firefox Add-ons For Web Developers you can check them out :

- Adblock Plus - Ever been annoyed by all those ads and banners on the internet that often take longer to download than everything else on the page? Install Adblock Plus now and get rid of them. You will never go back to browsing without it! (wiki)

- All-in-one Sidebar - One of my favourite Firefox 3 add-ons. Lets you quickly switch between sidebar panels, view dialog windows such as downloads, extensions, and more in the sidebar, or view source or websites in the sidebar. (wiki)

- DOM Inspector - DOM Inspector is a tool that can be used to inspect and edit the live DOM (Document Object Model) of any web document or XUL application. The DOM hierarchy can be navigated using a two-paned window that allows for a variety of different views on the document and all nodes within. (wiki)

- DownThemAll - DownThemAll lets you download all the links or images contained in a webpage and much more: you can refine your downloads by fully customizable criteria to get only what you really want. (wiki)

- Firebug - Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page. (wiki)

- Greasemonkey - Allows you to customize the way a webpage displays using small bits of JavaScript. (wiki)

- IE View Lite - It has a right click menu item to open a page in IE and a list of sites to always open in IE.

- NoScript - The best security you can get in a web browser! Allow active content to run only from sites you trust, and protect yourself against XSS and Clickjacking attacks. (wiki)

- Stumbleupon - StumbleUpon discovers web sites based on your interests, learns what you like and brings you more. (wiki)

- Tab Mix Plus - Tab Mix Plus enhances Firefox's tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more. It also includes a full-featured session manager. (wiki)

- Web Developer - Awesome array of web developer tools. Essential for any webmaster. (wiki)