Phoenix/Firebird/Firefox first made was in the 2002-2004 era where a substantial portion of the internet-trendsetting audience that adopted it in the US had broadband.

20% of adult Americans had broadband at home by early 2004 - https://www.pewresearch.org/internet/2006/05/28/part-1-broad... - which is not a majority but had heavy overlap with the group that wasn't just settling for IE6. Similar with Facebook - it was driven by the mostly-young tech-forward early-adopter crowd that either had broadband at home or was at university with fast internet.