There is a ftp site but one has to be sure they download the correct language version when doing so and make if it is 32bit or 65bit version as well.

https://ftp.mozilla.org/pub/firefox/releases/

Where can I get Firefox 52.9.0? There was a list of old versions here but now I can't find it. I found https://download2.portableapps.com/portableapps/FirefoxPortableLegacy52/FirefoxPortableLegacy52_52.9.0_English.paf.exe I really did not want the portable version but avast considers it a threat so it may not be genuine.

Portable Firefox has been around for a Very long time and and has been a safe download from their website links.

There is https://archive.mozilla.org/pub/firefox/releases/52.9.0esr/

However if you want to have both the old EOL Firefox 52.9.0 ESR and a current version of Firefox installed on Windows, then a safe way for less experienced Firefox users is to use the portable edition of Firefox 52.9.0 for when you need it along site the current Firefox. This way the Profiles you use will not share between the Firefox versions in any way.

The portable edition is self contained as it can even be run completely off say a usb flash drive.

There is a ftp site ... https://ftp.mozilla.org/pub/firefox/releases/

This is Not a ftp site as it used to be ftp://ftp.mozilla.org up til back in 2015 when Mozilla disabled the ftp:// protocol on their servers. Also Mozilla prefers we link to archive.mozilla.org