Old and outdated web browser versions have security issues that allow virus's and malware to infect your computer. They also don't follow the newest web development standards. If you're using an older browser version it increases your chances of allowing an intrusion through your browser and at the same time it decreases your positive experience.
For instance you might be using Firefox ver. 16, yet webmasters might be building web sites based on the the changes and web standards for Firefox ver. 46. In this case it's possible the page you're viewing on the Internet isn't rendering as designed and you could be missing out on web site content or experiences. Happy Browser supports the international movement to eliminate obsolete browser versions so that you can browse faster and view other web sites the way they were designed to be viewed.