Maybe you could do it slightly different though; have posts and the responses to them "folded" so only the original message shows on the page so that topics that do not interest one do not take up too much screen space? Then when you click on one it could "fold out" if you see what I mean.
I just did an upgrading from Phorum v3 to v5. The collapse command is moved into "my profile" in the new version. I've changed the default setting accordingly anyhow. Hope more comfortable now.