Do not enter your passwords, credit cards, or personal data into any browser version below Firefox 78 (released 2020). The "old version" you seek is a museum piece—admire it, emulate it, but do not daily drive it.
If you still need Firefox 4.5.0.1 for legacy software testing, comment below (or visit the MozillaZine forums) for specific FTP links. Otherwise, head to Firefox.com and download the modern version—your digital life depends on it. mozilla firefox 450 1 old version
However, the internet is full of typos, misremembered numbers, and third-party repackagers. The most likely candidate for your search is (also styled as 4.5.0.1) — a real, albeit ancient, beta or experimental build from late 2011. Do not enter your passwords, credit cards, or