While curly or smart quotation marks are perhaps more preferable when writing a novel or an official document, there is often a case for not automatically replacing your simple straight quotation marks (both single and double quotes) with these more fancy, curly quotation marks, like when you are including code in your text for example.
LibreOffice Writer by default does this replacement for you, but thankfully it at least makes it pretty simple to turn this feature off as well.
Simply access the main Tools menu, select the AutoCorrect options… and on the resulting dialog window, open the Localized Options tab.
There you will see sections detailing the replacement for both Single and Double quotes – simply uncheck the Replace box to turn off, or alternatively check the box to turn the autocorrect feature back on.
Easy as that! :)