• Engine: WebKit (until 27), Blink (based on WebKit, from 28 & beyond).
• Debugger: Dev Tool (integrated)

• Engine: Gecko (by Mozilla Foundation and the Mozilla Corporation)
• Debugger: Firefox Developer Tools (intergrated), Firebug.

• Engine: Presto                                                                      
  
• Debugger: Opera Dragonfly
  

• Engine: WebKit                                                                   
• Debugger: Web inspector (Integrated)

• Engine: Trident (also known as MSHTML)
• Debugger:
• IE (6, 7): ms script debugger, ms script editor, ms visual studio (IE dev toolbar)
  
• IE (8): developer tools
  

• Easy to use.
• Many plugin for Firefox browser, and many for firebug too. Ex: Cookies, Dust-Me Selectors, YSlow...
  
• Including HTML inspector, js debugger, network tool, DOM inspector...
  
• Can open extenal editor by "Open with editor" function
• Support lite version for the browser that not support Firebug (as) plugin.

• JavaScript command line
  
• Lists all kinds of messages and offers a profiler for JavaScript commands (including error, warning, debug message...)
  

• Displays the generated HTML/XML
  
• Right side it shows the CSS styles defined for the currently selected tag, the computed styles for it, layout information and the DOM variables assigned to it in different tabs. 
  

• Manipulate CSS rules (adding, editing and removing CSS styles of the different files of a page containing CSS)
  

• Debug JavaScript code                                                         
  

• Information about all the different DOM properties and methods.

• Monitor HTTP traffic initiated by a web page and simply present all collected and computed information to the user.
  

• The Cookies panel allows to display and manipulate cookies set by the current page. 

...available since Firebug 1.10 and was formerly a separate extension called Firecookie. 

• Bookmarklet (*)
  
• Live link: <script type="text/javascript" src="https://getfirebug.com/firebug-lite.js"></script>
  
• Local link: <script type="text/javascript" src="/local/path/to/firebug-lite.js"></script>
  

(*): (function(F,i,r,e,b,u,g,L,I,T,E){if(F.getElementById(b))return;E=F[i+'NS']&&F.documentElement.namespaceURI;E=E?F[i+'NS'](E,'script'):F[i]('script');E[r]('id',b);E[r]('src',I+g+T);E[r](b,u);(F[e]('head')[0]||F[e]('body')[0]).appendChild(E);E=new%20Image;E[r]('src',I+L);})(document,'createElement','setAttribute','getElementsByTagName','FirebugLite','4','firebug-lite.js','releases/lite/latest/skin/xp/sprite.png','https://getfirebug.com/','#startOpened');

• See everything in one DOM tree.
  
• Allows inspection and on-the-fly editing of DOM elements.
  

• Debug javascript
  
• Pretty Print javascript for debug
  
• View other resources
  

• Monitor HTTP traffic 
  

• Complete overview of where time is spent when loading and using.

• Profile the execution time and memory usage of a web app or page.
  

• Inspect resources that are loaded in the inspected page.
  

 

• Analyze a page for optimization