Firefox，Chrome插件

Firefox：


Firebug

    Inspect HTML and modify style and layout in real-time
Use the most advanced JavaScript debugger available for any browser
Accurately analyze network usage and performance
Extend Firebug and add features to make Firebug even more powerful
Get the information you need to get it done with Firebug.

HackBar

    This toolbar will help you in testing sql injections, XSS holes and site security. It is NOT a tool for executing standard exploits and it will NOT teach you how to hack a site. Its main purpose is to help a developer do security audits on his code. If you know what your doing, this toolbar will help you do it faster. If you want to learn to find security holes, you can also use this toolbar, but you will probably also need a book, and a lot of google :) 

HttpFox

    HttpFox is an addon for Mozilla based browsers (Firefox, Seamonkey, ...) that monitors and analyzes all incoming and outgoing HTTP traffic between the browser and the web servers. 

HttpRequester

    A tool for easily making HTTP requests (GET/PUT/POST/DELETE), viewing the responses, and keeping a history of transactions.

RestClient

    A debugger for RESTful web services.

    RESTClient supports all HTTP methods RFC2616 (HTTP/1.1) and RFC2518 (WebDAV). You can construct custom HTTP request (custom method with resources URI and HTTP request Body) to directly test requests against a server.

UnMHT

    UnMHT allows you to view MHT (MHTML) web archive format files, and save complete web pages, including text and graphics, into a single MHT file in Firefox/SeaMonkey.        

Web Developer

    强大的CSS调试和Form调试能力，对页面的分析非常全面，可以禁止页面的任一内联、文档、和外部CSS，可以直接编辑当前CSS，可以直接加载本地的CSS，可以用鼠标点击查看页面内任一元素的CSS，可以查看盒子模型结果，这还只是其功能一，其他的功能太多了，比如表单的POST和GET功能相互转换，检查XHTML和CSS语法，禁止一切可以禁止的，包括Cache、Cookie、图片、Java、Js、Referrer等等……如果你需要经常调试页面的话，这个插件能极大满足你的需求。