[nomel]

No, it was larger than many of the other browsers (like Opera), and size wasn't different enough to matter. Back then, download time was entirely quantized by "one night", because the only thing that mattered was that it finished by morning so someone picking up the phone wouldn't sever the dialup connection. A substation piece of software, like a browser, was happening during sleep time, regardless of size, and rare, so two night would also be fine (resuming was trivial with ftp, where these were sourced, usually from university mirrors).

[majormajor]

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.

[nomel]

Yes, 90% in 2002 and 80% in 2004 had dialup.

No, having broadband had nothing to do with desire back then. It was entirely based on availability and how quickly your local telecom/cable monopoly deployed it, being so bad that the government had to step in many times to motivate them. Everyone I knew purchased broadband (some cloning cable modem to get it for free) the day it was available. For broadband users, the difference in browser size was entirely negligible.

Facebook required that you were at a universities to register. Its not a reasonable thing to compare to web browser use.