If you are working with CSS or doing any web development and trying to tweak the design of a page then you will find the Firefox Web Developer add-in https://addons.mozilla.org/en-US/firefox/addon/60 extremely useful.
You can edit the CSS for your page in the Web Developer and see the changes taking effect as you make them. Changes are not persisted so if you refresh the page or navigate away from the page your changes will be lost. This means you can experiment with your layout to your heart’s content and always go back to what you started with if things don’t work out with the change.
It’s easy to cut and past the changes to your source files to persist any changes you want to keep after you have experimented with the layout in Web Developer.