The best way to upgrade your Firefox (and other programs) on Ubuntu is by Update Manager. You can go to dash screen (or hit win-key) and search for "Update Manager" or click the top right icon on the panel and find there information about updates. Once you open Update Manager it should search and display available updates. If not, try to find them manually by hiting "Search for updates" (or similar) button. Then just select updates you want, click install, type your password and wait while the updates are downloaded and installed. Firefox should be one of them. After Update Manager finish it's job, you should have the latest stable Firefox for your system.