From a developer's perspective, you can use these tools for testing to reproduce how an app or Web page will render in different browsers as well as on different devices and platforms through the responsive design and built-in simulators. The best part is that you can do all of this without having to leave your browser!
In addition to most browser makers focusing on the everyday user looking to surf the Web, they also cater to the Web developers, designers and quality assurance professionals who help build the apps and sites that those users are accessing by integrating powerful tools right into the browsers themselves. Firefox's Web Developer section includes tools for designers, developers, and testers alike such as a style editor and pixel-targeting eyedropper. Recently the Firefox team has lead the Dev Tools pack with tools for Grid,
Safari's diverse dev toolset reflects the large developer community that utilizes a Mac (OS X only) for their design and programming needs. In addition to a powerful console and traditional logging and debugging features, an easy-to-use responsive design mode and a tool to create your own browser extensions are also provided.