I use FireFox 80.0.1 on both Win7 64b and Win10 1909 64b systems.
I have NoScript, Ghostery, AdBlockPlus, PrivacyBadger extensions all enabled.
My computers are on a home network and have both IPv4 and IPv6 addresses enabled.
I use Xfinity cable internet thru an ASUS access point (in my case wired) at about 300Mbps rate.
I have no problems downloading files from the Intel FPGA download site.
In fact I just downloaded 19.1 full set of files yesterday successfully and will install shortly.
I am using 19.1 because I still want/need CycloneIV support.
So YMMV.