There are many ways to define what is best, but as quantified by overall speed, the ability to customize, security, ease of use and compatibility, Firefox comes out on top. The age of Windows XP makes it unable to handle many of the updated features of the latest browsers, but Mozilla Firefox gives the most complete version of the Web possible on this ancient operating system.
Features has an incredibly diverse amount of extensions that tweak its functionality and aesthetics exactly to the user's liking. Its HTML5 support allows it to feature the latest media and interactive content. Many of the former issues that Firefox had dealing with memory leaks and crawling speed were eliminated in 2013, making this browser second to none.