Addon:MeasureIt
Latest Version:0.4.10 by Kevin Freitas
Version Status:Fully Reviewed
Install Link:Install
Summary:

Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.

Addon:FoxyProxy Standard
Latest Version:3.4 by Eric H. Jung
Version Status:Fully Reviewed
Install Link:Install
Summary:

FoxyProxy is an advanced proxy management tool that completely replaces Firefox's limited proxying capabilities. It offers more features than SwitchProxy, ProxyButton, QuickProxy, xyzproxy, ProxyTex, TorButton, etc.

Addon:Firebug
Latest Version:1.9.1 by Joe Hewitt
Version Status:Fully Reviewed
Install Link:Install
Summary:

Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page...