hmm, "Falló la descarga. Comprueba tu conexión.". - the same problem.
I have Windows 10 Home, Kaspersky Standart 21.18.5.438 (a), Mozilla Firefox 130.0.1 (64-bit)
other addons are ok
I'm in Ireland and when I try to install Kaspersky Protection 2021 addon into the latest Firefox I get the message: "Download failed. Please check your connection."