Since the standard Firefox homepage will only offer the latest (incompatible) version, you must access the official Mozilla archives:
To download Firefox for Mac 10.9.5 (OS X Mavericks), you must use a legacy version, as the current release no longer supports this operating system. The final compatible version is . Step-by-Step: How to Download and Install download firefox mac 10.9.5
: Drag the Firefox icon into the Applications folder. Since the standard Firefox homepage will only offer
: Open Firefox from your Applications folder. When prompted, click Open to confirm you want to run the application. Important Considerations for OS X 10.9.5 you must use a legacy version