1. Firefox Developer Tools
2. Chrome Developer Tools
3. Web Developer
Web Developer is an extension for Firefox and a few other browsers which adds a toolbar with a lot of options and features for debugging and inspecting web pages. This tool is especially helpful for working with large CSS files and projects that you may be unfamiliar with. It even has a built-in feature for validating the syntax of the page and help you locate the possible errors.
4. Safari Developer Tools
Make sure that your website works well with all the major web browsers using Safari Developer Tools. Safari comes equipped with Web Inspector, a powerful tool that simplifies the job of modifying, debugging, and optimizing a website for optimal performance and compatibility on different platforms. Its responsive design mode helps you get a preview of your web pages for various screen sizes, orientations, and resolutions.
5. Internet Explorer Web Edge (Developer) Toolbar
Fiddler is an extension of Internet Explorer that lets you debug web applications and analyze a web page’s HTTP traffic. It lets you set up breakpoints and has a lot of other features useful for debugging. This tool is quite extensible and even lets you create your own scripts to perform useful functions.
7. Open Dragonfly
DebugBar is a free tool for personal and educational use that helps you in debugging. It is an in-browser extension for Internet Explorer. You can send a web page screenshot via email and view both the original and interpreted code using this tool. It also has a color picker and a Console API. It helps you gain some quick and easy information using a command line interface.
YSlow is a tool that helps you analyze web pages in order to understand why they are running slow. It works on Yahoo’s rules for high-performance websites and displays various relevant statistics for a page. Apart from that, it also gives suggestions for improving the performance of web pages and summarizes the page’s components.
HTTPWatch is yet another traffic viewer and debugger, quite similar to Fiddler in its working and approach. Generating request level time charts, decryption of HTTPS traffic to assist debugging, exporting captured data to XML and CSV formats are some of the prominent features of HTTPWatch. Its basic edition is free, but it has a professional and advanced version, too.
11. Live HTTP Headers
Live HTTP Headers is actually a Firefox extension that lets you inspect HTTP request and response headers. You can debug your web applications by exploring the HTTP headers with the help of this tool. Apart from that, the tool helps you obtain information about the website’s server and analyze cookies sent to the client while requesting a particular page.