Firefox no longer works with Mac OS X 10.5
Firefox 16 is the last version of Firefox that works with Mac OS X 10.5 (Leopard) on an Intel-based Mac. To stay safe online, we recommend upgrading your computer or operating system. This article covers your options.
Option 1: Upgrade your operating system
Apple stopped supporting Mac OS X 10.5 in 2009 and many software makers have either dropped support for it or soon will. Newer versions of Firefox require OS X 10.6 (Snow Leopard) or above. Depending on the age of your computer, you may be able to upgrade to a newer version of OS X. If you can't upgrade to OS X 10.9 (Mavericks), newer versions of Firefox will also work on 10.8 (Mountain Lion), 10.7 (Lion) and 10.6 (Snow Leopard). Apple still supports these older operating systems, although it's likely that it will drop support for 10.6 (Snow Leopard) soon.
Option 2: Switch to a newer computer
If your computer is not capable of running a newer version of OS X then you will need to purchase a new computer to allow you to run the latest and greatest Firefox. You can transfer your settings across so you don't lose anything.
Option 3: Continue to use Mac OS X 10.5 and Firefox 16
If you can't switch to a newer computer or upgrade your operating system, you can continue to use Mac OS X 10.5 and Firefox 16. However, you will no longer receive stability and security updates available in upcoming Firefox versions. See the Security Advisories for Firefox for a list of vulnerabilities fixed in past releases.
If you need to reinstall Firefox 16 on your Intel Mac, you can download it from this link: