Alternatively, I found this site below which, contrary to what the URL suggests, still offers the main Firefox 56 browser in *various* OS not just Windows:
https://www.softexia.com/windows/web-br ... firefox-561. Install and then quickly (temporarily) disable your wifi before Firefox can auto-update to v57.
2. Then go into Firefox's Preferences and change to 'Never Update'.
3. Turn your wifi back on.
4. Then reinstall Property Bee as normal and it works for me (Mac version).
Of course you have to live with the potential security risks of having a non-up to date browser on your system. Offset this by only using Firefox 56 for Rightmove and Property Bee but nothing else. Do all your other web browsing in another browser which you then keep up to date with all the usual security and version updates.