I have Firefox esr on a Raspberry Pi and Firefox 83 on windows, both work fine using ~ shop.fairphone.com/gb_en/fairphone-3-plus ~ so it is probably some odd setup you have.
Just updated to Firefox 84 on Win 10 and the link works fine.
Reporting some potentially useful debug info:
For me, the bug is still present. I was unable to place an order for a FP3 for delivery in the UK (in chromium) even after changing the region explicitly to UK.
However, on Firefox the explicit region change worked, and I was able to place an order.
Nothing to do with Brexit! It’s an issue with cookies. Deleting stored cookies for fairphone.com should resolve the issue.
To delete cookies in Firefox, click on ‘Open Menu’ hamburger menu in top right - Preferences - Privacy & Security - under Cookies and Site Data click Manage Data - type ‘Fairphone’ in the Search Websites box and press Enter - then click on Remove All - restart browser and you should be good to go.