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Everyone already knows and loves Firebug and Web Developer. So instead of another redundant “Top 10” list, here’s a collection of 15+ great add-ons that you might not have heard about yet.

Dust-Me Selectors

Dust-Me Selectors helps you find unused CSS selectors. You can use it to test an individual page, or let it spider your entire site. Results are presented in a nice color-coded report and can be exported as CSV.

Alternative : CSS Usage

Inline Code Finder for Firebug

Finds and highlights HTML elements with suspicious hacks like inline JavaScript events, inline styles and javascript: links. It also presents a summary report and calculates an overall score based on how many such elements were encountered on the page.

Window Resizer

Lets you quickly resize your browser window to a number of common screen sizes so that you can see how you page would look at a different display resolution.

Alternative : FireSizer

RESTClient

RESTClient is a very handy tool for web service developers, allowing you to directly test and analyze RESTful APIs. It also includes partial support for the WebDAV protocol.

JSONView

Lets you view JSON data directly in the browser and supports syntax highlighting and code folding. Overall, it’s another simple yet useful extension for developers working with WEB APIs.

Remove Cookie(s) for Site

Remove all cookies from the current site with a single click. If you need something more advanced, check out the alternatives below.

Alternatives : Add N Edit Cookies, FireCookie

CookieSwap

Lets you quickly swap all your cookies so that you can easily switch between different user accounts without logging in/out all the time. While this add-on is primarily advertised as a way to manage multiple webmail/Amazon/Google accounts, it can also be useful to web developers – e.g. for testing how your site behaves from the point of view of normal visitors vs. logged-in users, or users with different privilege levels.

TamperData

With TamperData, you can intercept HTTP(S) requests made by Firefox and view/modify their headers and POST data. All requests are logged and can also be exported as XML or displayed on a graph. Handy for security testing and catching certain breeds of elusive server-side bugs.

Alternatives : Modify Header, UrlParams

View Dependencies

Essentially a simpler alternative to YSlow, View Dependencies adds a new tab to the Page Info window, listing all the files that were loaded to display the current page.

HackBar

HackBar is a large collection of simple penetration testing tools, useful for security-auditing your site and testing it for common vulnerabilities. You can convert strings to hex and back, encrypt text with SHA-1, SHA-256 or ROT13 (if only just for fun), try a number of typical SQL injection/XSS tricks, manipulate POST data, spoof the referer, and much more.

Related : Access Me, XSS Me

iMacros for Firefox

“Whatever you do with Firefox, iMacros can automate it.” It can automatically fill forms, download & upload files, scrape web pages, memorize passwords, and so on. iMacros really makes tasks like web application testing, data mining and mashup creation much easier.

TableTools

Sort, filter or copy any HTML table. You can filter each column either by selecting a value from a drop-down menu or by searching for a specific keyword (regular expressions are also supported). TableTools will automatically recognize frequently used data types like numbers, timestamps and IP addresses, and sort/filter them intelligently.

SQLite Manager

Manage any SQLite database on your computer. This add-on has all the features you would expect from a full-featured DB manager – you can view the database structure, create, modify and search tables and views, manage indexes, export and import data, and manipulate pretty much any other aspect of the database.

Pencil

Make diagrams, build GUI prototypes and sketch website mockups with this awesome Firefox add-on!

Flashbug

A Firebug extension that logs all the SWF trace output to the Firebug console. Can be invaluable for debugging Flash-based applications and games.

FireLogger

Debug your server-side Python and PHP code from within the browser. After installing this Firebug extension and the appropriate Python/PHP library, you will be able to send logging information from your server-side scripts directly to the FireBug console.

Alternative : FirePHP