I'm not sure I understand your problem, but it could be that you're calling "conversations" what Thunderbird calls "threads". Look at the options available under View > Sort by. There are three options for threads at the bottom there. I guess you have Threaded ticked, so choose Unthreaded instead.

I want to put off threads but the option View > Sort by is not in my menu I have the menu-option view (beeld) but then only 3 options toolbar,styling, folders and no :sort-by" option What to do to get the option sort-by?????

Really? You don't see a Sort by option further down in the View menu? Check the following article (in your language) and see whether you can find the way to change your threading options there:

https://support.mozilla.org/kb/message-threading-thunderbird