This is probably happening due to an addon. I did some testing and that was my observation, using Thunderbird and Notepad++

Not sure I understand you there, david. It was you observation it was due to an addon that double line breaks appear? In notepad++ I have no plugins that could cause this. Btw. Also notepad.exe show double line breaks

My experience with Thunderbird, copying text to Notepad++ shows single line breaks. My inference is that you may have an addon in Thunderbird that is inadvertently causing this.

I have not so I assume you have a plugin installed in notepad++ that prevents double line breaks.

Also in windows notepad I see double line breaks as well. So the question remain

how do I copy paste code sections from Thunderbird to a text editor without double line breakes?

I use Notepad++ with no addons or plugins and have no problems. You might try help>troubleshoot mode to temporarily stop any addons and see if that works.

It happens when content is styled with the preformatted style in thunderbird

That also works for me. There is something in your setup that is causing this.