These firefox addons will make your web designing life easier definitely. It can do unbelievable things you can ever expect
Rainbow Color Tools
It has Color picker and eyedropper + saving colors and trying out colors with drag and drop.
Back link check for a domain or URL, to analyze the off page SEO.
Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse.
View Source Chart
It inspects the DOM at lightning speed and you can check it at a glance or can zoom it for better view
It features an advanced accelerator that increases speed up to 4x and it allows you to pause and resume downloads at any time.