DOM Inspector is a tool that can be used to inspect and edit the live DOM of any web document or XUL application. The DOM can be navigated using a two-paned window displaying a variety of different views on the document and all nodes within.
For extension developers: allows automatically adding or updating browser extensions, e.g. with command line tools.
Pretty-prints JSON content in the browser for easy, unobtrusive viewing.
Easiest way to Add and Manage Custom User Styles in Firefox.
Add, Edit, enable, disable and delete in-numerous User Style Sheets to redesign the Firefox and the Web.
Install Browser Extensions straight from GitHub Repositories
A suite of tools for developers.
Foundstone HTML5 Local Storage Explorer allows for viewing, modifying and deleting of data items stored in the browser's LocalStorage. LocalStorage is a client side storage technology introduced in HTML5.
Markdown document viewer.
Allows to visualize formatted markdown documents in browser.
A quick restartless magnifier to debug styles and web pages.
Create and manage add-ons made from Greasemonkey and Scriptish user scripts
CSSsir is easy and handy tool which allows you to generate cross-browser code for main CSS- rules in a few clicks.
This CSS3 generator will be useful not only for web-masters and designers but also for common users.