You can use the following direct links from the official archive.mozilla.org site:
This browser can only be found in Mozilla's official public archive. When downloading, it's important to use only archive.mozilla.org or ftp.mozilla.org to avoid modified or unsafe versions from third-party sites. firefox version 52.9. 0 esr download
Firefox Version 52.9.0 ESR Download: A Complete Guide to the Legacy Browser You can use the following direct links from