Most modern browsers such as IE9, Firefox, Chrome, Safari, and Opera provide developer tools including console tools, CSS/HTML/DOM inspectors, a JavaScript debugger, a network or HTTP traffic inspector and more. Many of these are useful for Ext.NET applications as well.
Most development tools are native to their browser. Being native means they generally perform quite well even on complex web application pages. Firefox has the well-known Firebug extension. It is very rich and powerful. Although Firefox has recently started providing native tools, as of writing they do not match Firebug fully. Firebug is also perhaps the most powerful for Ext.NET debugging. So, although it could be a bit slower to run if your page is extremely heavy or you leave Firebug on all the time, it is usually fine, but it may be something to be aware of especially if your development machine is old or doesn't have enough CPU or memory.