Not only is Firefox a good browser, but it has some very useful development tools.  The Web Developer plugin is an invaluable tool when it comes to building web pages.  It has many useful features.  Highlighted below are some of the ones we use most often:

Edit CSS

This feature allows you to edit the CSS for a page inline.  This is extremely useful when trying out new styles on your pages.

Display Block Size

 This feature allows you to view the block sizes on the page.  This is very helpful for CSS based themes that use fixed width blocks.

Display ID/Class Details 

  This feature allows you to view the block sizes on the page.  This is very helpful for