There are hundreds of Add-ons for Firefox, most of them are not suitable for helping testers whilst testing but amongst the vast number available there are some true time saving gems.
Here’s a few that I use on a regular basis, you might also find them quite useful.
1. Selenium IDE
https://addons.mozilla.org/en-US/firefox/addon/2079
Selenium is fast becoming the standard web test automation tool and there is good reason for it. You can use it for record and playback testing or get down to code level and drive it that way. The browser platform for replaying the tests is not complete yet but it is getting there. The Selenium IDE is the basic record and playback tool which is invaluable for automating website pages or for quickly loading data through the UI.
2. IE Tab
https://addons.mozilla.org/en-US/firefox/addon/1419
IE Tab is great if you use Firefox as your default browser, especially if you use sharepoint sites as you can flip the current tab to render in IE. In sharepoint this enables controls that are simply not available in Firefox and displays the pages correctly in some instances. It is also great for quickly seeing how a site will look in IE whilst you are testing, although it should not be used to substitute full IE testing.
3. Firesizer 0.65
https://addons.mozilla.org/en-US/firefox/addon/5792
This is a handy little tool that simply resizes the window to a predefined size. Great for testing how your site will look in different sizes.
4. W3C Page Validator
https://addons.mozilla.org/en-US/firefox/addon/2250
Great tool for checking your site against the W3C conformance standards. Does sometimes complain about things that are not W3C failures and does require you to validate each page in turn, but still useful.
5. Pencil Sketcher
https://addons.mozilla.org/en-US/firefox/addon/8487
Amazing little add-on which allows you to design screens and mockups really quickly and within the browser. You can add all sorts of shapes and widgets. Brilliant for mocking up screens for the test plans and cases.
6. SQL Injection
https://addons.mozilla.org/en-US/firefox/addon/6727
This is a great tool for checking SQL injection vulnerabilities on sites under test. It generates the SQL for you and adds it to each available input field.
7. Quick Restart
https://addons.mozilla.org/en-US/firefox/addon/3559
This is a very simple tool but invaluable. It basically adds a restart button to the tool bar which simply restarts the Firefox browser. Invaluable when testing cookies and sessions where lots of restarting is needed to clear them down.
8. Firebug
https://addons.mozilla.org/en-US/firefox/addon/1843
Brilliant add-on for inspecting the page under test. You can view the HTML and CSS as well but the inspection tool is very useful.
9. Molybdenum
https://addons.mozilla.org/en-US/firefox/addon/4149
This is a cool little app for managing Selenium tests. It gives a clearer interface for managing suites and cases. Still not perfect though but better than the standard Selenium IDE.
10. Regular Expressions Tester
https://addons.mozilla.org/en-US/firefox/addon/2077
Useful tool for testing Regular Expressions.
These are just a few of the useful tools that can be found to help testers be more efficient and effective whilst testing web apps.