usefull add-ons for markup engineers. also recommended to install PrefBar, Page Speed, my collections (Base Pack, Web Designer's Pack).
FoxyProxy is an advanced proxy management tool that completely replaces Firefox's limited proxying capabilities. For a simpler tool and less advanced configuration options, please use FoxyProxy Basic.
View HTTP headers of a page and while browsing.
YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
The extension generates a documentmap or index of any web document structured with headings and/or with sections in HTML 5. It shows the headings structure, the errors in the structure (ie. incorrect levels), and it works as HTML5 Outliner too.
Calculator has an interface similar to Microsoft Powertoy Calculator and it offers great features like history, conversions, many functions... And it's a great open source replacement to the calculator included in Windows accessories!
The main goal allows users to open (and in some cases edit) page source with external applications.