Given the security risks associated with using outdated software, consider these alternatives:
Firefox 3.6 only supported TLS 1.0 by default. Webservers today require TLS 1.2.
Before installing, ensure your operating system can handle the application architecture: Operating System Compatibility Notes Fully supported natively. Windows 10 / 11 Runs via Compatibility Mode (32-bit installer). Mac OS X
Mozilla maintains a public archive of almost every browser version they have ever released. This is the safest and most reliable source.