I had some problems in front of loooong threads on flarum: in a forum without pagination (which is a plus, plus also bring new problems), it becomes really difficult to navigate into threads with 300+ messages, and to find the message you were looking for. So here are two suggestions...
A first solution could be to be able to bookmark messages inside the threads, and to be able to access them without leaving the thread.
It could take the form of a little "bookmark" button on the bottom in each message, button which would create a "chapter"/"mark" on the scrollbar. For instance, if you bookmarded the 180th, 360th and 450th message of a 566 messages thread, it could look like this:
Or like anything else more ergonomic, but just in a way which could make you able to click on the mark to reach instantly the message you're looking for inside the discussion.
As I can't test it on freeflarum, I'm not sure how much what i propose here is close to that extension from @clarkwinkelmann : https://discuss.flarum.org/d/27698-post-bookmarks
The second solution could be to have search bar inside the threads, to find quickly a message, or just a moment of the discussion, while staying inside the thread. One can indeed not use the searchbar of the browsers, as the discussion is not fully loaded until read entirely in detail.