FireFox Tools

ieview
Just right-click on a page or link and select it to open in Internet Explorer. I use this one a lot, especially when working in sections where I know there will be a great possibility that I’ll need to hack it to work in IE.

View Formatted Source & View Rendered Source 
View Formatted Source will even import referenced files, like stylesheets, and display them in a box along with the source. It will also show you which css styles are affecting an element; very cool. I’ll add a third one just to be confusing: ViewSourceWith is a great little tool for sending page markup to an external editor, such as Dreamweaver or TextMate.

ColorZilla – Grab colors from the page with an eyedropper tool built into your browser, along with a color-picker and zoom tool. Another cool feature of this one is its ability to outline elements on the page as you glide over them.

MeasureIt – Especially helpful when You have a space to fill with an image and would like to quickly get the dimensions of that space. Just resize the box by dragging the tool across the space and you’ve got size.

HTML Validator & Checky – Checky is really helpful in quickly accessing various online validation tools, like if you need to check your css for typos. The HTML Validator is great because it sits in the bottom right corner of the browser and tells you if the page source is Tidy, and will tell you exactly what’s wrong with it when you click the icon.

LinkChecker – Need to check the links on a page in one fell swoop?

RiteOfTongue  For  bad spellers out there.  When you’re entering text into a form (like an html rotator in BV) you can right click a word or phrase for spelling suggestions.