Reply to: by sporobolus

it’s not the threads, it’s how they are displayed; Thunderbird (as an option) uses the in-reply-to headers to show who replies to whom in a hierarchical display, showing the real flow and branching of a conversation,, while Gmail (and its imitators such as OS X Mail) simply shows a flat list in date order, scrambling the context and making threads less conversational; unlike Gmail, Thunderbird also displays subject lines within a thread when they are changed to denote tangents