6 Must have plug-in for Firefox

[pow-wow]

* Web Developer
Menu and a toolbar with various web developer tools.
* SeoQuake plugin
Show indexed pages, pr, backlinks, dmoz, alexa rank, if domain is present in archive.org, internal link, external link, keyword density and much others thing of visited website
* iWeebTool plugin
Access webmaster services and web tools from iWEBTOOL directly from the web browser
* Firebug Plugin
You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
* ColorZilla
This plugin is very useful when you want to set in the right mode the colors of your adsense
* MeasureIt
Draw out a ruler to get the pixel width and height of any elements on a webpage.

[Gorkfu]

I would also recommend Total Validator: The all-in-one validator.
It checks your code to see if it's compliant with the doc type you use.

[pow-wow]

Thanks Gorkfu for the addition, i willl download that too.

[Arleigh]

I would also add these plugins:

Google Toolbar - Google Toolbar for Firefox
All-in-One Sidebar - https://addons.mozilla.org/en-US/firefox/addon/1027

[alec11]

I like SeoQuake plugins, thanks for sharing here.
Just remember, most firefox plugins here is not compatible with new version firefox, firefox 3

[headdown]

Quite like Rank Checker from seobook.com, though still trying it out. Slight problem with FF3 having compatibility problems with some of the good ones but they are usually updated quickly.

[pow-wow]

Linkdiagnosis.com - It check Back link with all the data such as :
* Google Pagerank of every backlink
* Anchor text of every backlink
* if links are no-follow
* most popular anchor texts
* charts with breakdown of pagerank and link types

[seo_mike]

coll man,

good post. seoquake rocks

[MarkeD]

I would be careful with overusing some of these plugins though, since too many requests to Google for PR, links etc will flag your IP up as a possible bot and they may block your searches with a Turing test.

[eugen]

Did u mentioned FireBug ? Also some FireBug add-ons: FireCookie, YSlow