Firefox
From MoodleDocs
What is Firefox?
Firefox is a free internet browser owned by Mozilla.com.
You can download it from http://www.mozilla-europe.org/en/products/firefox/
Firefox provides a number of very useful developer tools, such as Firebug, the Web developer toolbar, Stylish, and DOM Inspector. These are very useful when investigating and fixing bugs in Moodle, and when working on themes.
Add-ons
You can customize Firefox with a host of add-ons.
Search plugins
A search plugin allows you to search certain websites directly from the search bar of the browser. See Firefox search plugins.