Firefox is a Browser not a email program. So what email service are you talking about here?

Thunderbird threads conversations by default. Many people don’t want threaded conversation. There are several things one can do to prevent and change threaded conversations. First, go to Settings - General - Config Editor. Search for mailnews.default_view_flags and change the setting to 0. This setting should change the default view. If threaded conversations do appear, there are at least two ways to change them to unthreaded. 1) View - Sort by - Unthreaded affects the current message list. 2) Copy your preferred message list settings from one list to another. At the far right of the column headers in every message list, there is a button to “Select columns to display”. Click on it. At the bottom of the list that appears, there is a command to “Apply current view to …” If you have folders with “children” folders, you can apply your preference of no threads to the parent and the children in one step. Official documentation on threading is here: https://support.mozilla.org/en-US/kb/message-threading-thunderbird '''SORTING''' Once you have your columns set as you want them, you can use the next-to-last option in “Select columns to display” to copy your settings to other message list windows. To set the default sort column and sort order, use the configuration editor (Settings - General - Config Editor …”.) To get the sort order typical of e-mail apps, descending by date, set mailnews.default_sort_type to 18 (date) and mailnews.default_sort_order to 2. More information about sorting options is here: https://support.mozilla.org/en-US/questions/143239