Let's keep in mind for a moment that phpBB3, having been released in late 2007 and since then largely only been given functionality and security patches, not modernity upgrades, isn't really a shining example of the modern Internet. In fact, I would argue that it's barely different in overall functionality than phpBB2 and other really old bulletin board and other early web applications, such as Snitz, YaBB, etc.

On the actual topic: Back in the day, I almost universally had the best luck with Internet Explorer 3, or 4 if the system could possibly support it. This has basically held true for me all the way out through IE 5.2 on the latest OS 9 releases. Mozilla worked, but I never quite wanted to use all of its functions at once, and it always wanted way more memory than any system I owned at the time period had.