Firefox 3.6.28 is the last version of Firefox that works with Mac OS X 10.4 (Tiger) or with Mac OS X 10.5 (Leopard) on a computer with a PowerPC processor. To keep your personal information safe, we recommend you switch to a newer computer capable of running OS X Yosemite (10.10) if possible. This article covers your options.
Apple stopped supporting Mac OS X 10.4 (Tiger) in 2007 and support ended for Mac OS X 10.5 (Leopard) in 2009. Many software makers have either dropped support for these older systems, or soon will. Your best bet is to get a computer capable of running the latest version of OS X - Yosemite (10.10). This is the most secure option and will allow you to run the latest and greatest Firefox.
If you can't switch to a newer computer, you can continue to use Mac OS X 10.4, or Mac OS X 10.5 with a PowerPC processor, and Firefox 3.6.28 - they will continue to work. However, you will no longer receive stability and security updates for Firefox. See the Security Advisories for Firefox for a list of vulnerabilities fixed in past releases.
If you need to reinstall Firefox 3.6.28, you can download it from this link: