That Firefox S3 bucket is slow to download from on 2degrees 100Mbps fiber, that I know is good.
However Firefox nightlies from their servers are fast
Oregon S3 (54Mbps and rising)
Sydney S3 (68Mbps and rising)
Sometimes downloads are fast. Sometimes downloads are slow. It could be server capacity, bandwidth, congestion, routing, any number of things. Sometimes the ISP can mitigate, often not.
I can verify, I get the same results from the same sources.