please help to me download this

You mean you want to view and use the web whatsapp site in the Firefox web browser? The most common issue is due to using a old version of Firefox as starting last November they have required at least Fx 115.0 or later. I guess this was done to accommodate the Windows 7, 8, 8.1 and macOS 10.12, 10.13, 10.14 OS users as they cannot use Firefox 116.0 or later. https://support.mozilla.org/en-US/kb/update-firefox-latest-release