I don't remember how the program behaved on Windows, but on Linux pressing Enter for a feed item (with a Website field) always opens the Thunderbird's internal browser, while clicking the URL launches my default browser (Firefox).
This damages my keyboard-centric workflow.
In the Thunderbird's config editor I've set both network.protocol-handler.warn-external.http and network.protocol-handler.warn-external.https to true and associated Firefox in the 'attachment' options (reference). This made no difference I was able to notice.
rss.show.content-baseandrss.message.loadWebPageOnSelect. It's been ages since I used TB for RSS but my notes say these entries helped me. I cannot be 100% sure what you need is what I had at that time, but it can't hurt to try... – Sep 08 '20 at 11:27rss.show.content-baseto3alone appears to have started loading websites in my default browser. While this doesn't disable the TB's web browser (which I'd prefer for security reasons) and the changelog for instance loads there, I consider the comment as an answer. – user598527 Sep 08 '20 at 14:46