Twenty Essential Firefox Add-ons For Testing

01. Firesizer 1.2
Provides a menu and status bar to resize the window dimensions to a specific size. Unlike other similar extensions, this one sets the size of the *entire window*, not just the HTML area. This add-on is extremely useful if you want to test how your application will look in different size windows.

02. W3C Page Validator 3.0.0
Validates a page using the W3C Markup Validation Service. Adds an option to the right-click context menu and to the Tools menu to allow for easy validation of the current page. Opens the results in a new tab. This is a simple extension that will work only for online pages. Depending on your context, if your organization is committed to create W3C compliant web applications, this might be very handy.

03. SQL Injection 1.2
This is an excellent tool to help developers in identifying SQL injection vulnerabilities. This add-on transform checkboxes, radio buttons and select elements to an input text box. It sets all form fields free to edit their values.

04. QuickRestart 1.1.6
If you need to restart firefox to test your cookies, sessions or because of any other reason, this little button in your toolbar will do it with a mouse click. This simple extension adds a "Restart Firefox" item to the "File" menu. You can also use the Ctrl+Alt+R keyboard shortcut, or the included toolbar button. To use the toolbar button: right click on toolbar -> Customize... then drag the Restart button to the toolbar. This is a very simple utility, but can be extremely useful.

05. Firebug 1.7.0
Firebug integrates with Firefox to put a wealth of web development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page. Your firefox should have firebug if you are working in web application testing domain.

06. Regular Expressions Tester 3.2.11
This small add-on allows you to test your regular expression. The tool includes options like case sensitive, global and multiline search, color highlighting of found expressions and of special characters, a replacement function incl. backreferences, and auto-closing of brackets, testing while writing and saving and managing of expressions.

07. HttpFox 0.8.8
It is a HTTP analyzer addon for Firefox. This add-on is an amazing tool if you want to dig deeper into http request / response and analyze all the traffic. Using this add-on, you can get information about request / response header, sent and received cookies, Querystring parameters, POST parameters and Response body.

08. HackBar 1.6.0
It is a Simple security audit / Penetration test tool. It is useful for SQL injections, XSS holes and site security.

09. Web Developer 1.1.9
The Web Developer extension adds various web developer tools to a browser. This one along with the firebug are must have extensions if you are working in web application domain. These extensions are complete toolset and probably require separate articles to explain their capabilities.

10. Accessibar 0.7.8
It is an amazing add-on to test accessibility of any site. Using this add-on, you can change font size, line spacing, hide all the images / flash and so on. It also integrates text to speech reader and reads string on mouse hover / focus elements.

11. Add N Edit Cookies 0.2.1.3
There are very few applications which does not use cookies these days. This little add-on allows you to add / edit session, saved cookies and test how your application will respond to changes in the cookies setting.

12. LinkChecker 0.6.7
This is a very simple tool to check if there are any broken links on the web page. It highlights all the links in various colors to show if they are broken or not.

13. MeasureIt 0.4.7
If you have a design team which is very particular about the size of every element, than this little add-on can be used to test height and width of all the elements. It draws a ruler around any element and shows height / width of the element.

14. YSlow 2.1.0
YSlow analyzes web pages and gives you information on why they are slow based on the Yahoo's rule for high performance websites.

15. User Agent Switcher 0.7.3
If your application behaves differently based on the user agent or if it records this information than this add-on can be very handy. It simply switch the user agent of the browser. Please note, changing user agent will not make it render like IE.

16. FireShot 0.88
FireShot can be handy in situations where you want to take screen shot of your web application. FireShot is a Firefox extension that creates screenshots of web pages (entirely or just visible part). It even allows you to write comments, highlight specific element of your application and so on.

17. URLParams 2.2.0
This little add-on make it convenient to analyze GET and POST parameters of the current website in the sidebar. You can ever change their values, add new parameters etc.

18. Tamper Data 11.0.1
Tamper Data allows you to view and modify HTTP/HTTPS headers and post parameters. You can use it to test security of your web applications by modifying POST data and analyzing how your application will respond to those changes.

19. View Dependencies 0.3.3.2
View Dependencies adds a tab to the Page Info window, in which it lists all the files which were loaded to show the current page. This is an extremely useful add-on if you want to figure out what are all the files / images etc. are getting downloaded for every page, how much time they are taking and many such details.

20. Flash Switcher 2.0.2
If you are involved in testing Flash Applications, than this little add-on is extremely useful to test your flash application on various versions of flash players. This extension allows you to easily switch from one flash player plug in version to another, or also to remove or save the currently installed plug in (maybe for testing the express install).

Reference - http://www.testinggeek.com/index.php/testing-articles/176-twenty-essential-firefox-addons-for-testing