A more popular forum these days is discourse, which is everything
classic web forums are not, including being mostly javascript and
having the endless scroll anti-feature for pages and post threads.

Discourse also has an annoying habit of complaining about older or
niche browsers with an "unsupported" message, coupled with a bare
forum view that lacks even basic features like search, login and
posting.  Basically, if it's not a recent version of Firefox,
Chrome, or Safari, fuck you. Even the latest version of Pale Moon
doesn't meet the standard, and this is not just a check of the
client's user-agent - this message is generated by a bit of
javascript that checks for certain browser features.

Thankfully, it's easy enough to block this nonsense with ublock
origin (or most other ad-blockers). Just add a custom filter to your
blocklist:

    */assets/browser-detect-*

With this filter installed, any discourse-based forum will work fine
in Pale Moon, and I'm sure any other javascript-supporting
browser. Whatever feature discourse is checking for is not essential
to using the forum, but even if it was - please let the user decide
whether or not to continue using the forum or not.