The OP couldn't find the relevant firmware anywhere else.
Since that company is an authorized Dahua distributor it is safer to get the firmware from them than, for example, from Andy who is not an authorized Dahua distributor or download any firmware posted on this forum. No offense to anybody, just explaining the logic.
If Dahua signed their firmware this conversation wouldn't have happened. Does anybody know if Dahua signs their firmware? I see a file "sign.img" in one of their bin packages but I don't know if it means anything. The browser plugin from the latest firmware I tried was digitally signed. It's a good sign.
EDIT: Just found in "
Cliff Notes": "Dahua started signing their firmware in 2017 (varies by model)". So the trust in the website is irrelevant. A tampered with firmware simply won't install.