Until now, I admit I was not a fan of Firefox. Of course I use it because a significant portion of users now have it as their main browser, so I have to check my projects works fine with it.
But now I discovered a new plugin for Firefox (and free) and now I am using Firefox as a developer tool.
Web Developer extension is an incredible tool. You can see every details on your web page, edit the CSS, check for broken links, validate your page, view all images details, and much more.
I really like to edit CSS and see immediately the changes !
I used for IE some similar stuff before, but not as complete as this extension.