I am using Thunderbird 115 with mostly default settings. When I compose a brand new message, pressing the Enter key inserts a new paragraph, with extra line spacing, and pressing Shift+Enter inserts a line break with single line spacing. This is how I want things. I am experiencing a problem when replying to emails from a particular sender. Their emails HTML code contains a "<style>" element that removes the extra spacing for paragraphs: <style type="text/css" style="display:none;">P {margin-top:0;margin-bottom:0;}</style> When replying to their emails, their HTML is inserted as quoted text, and the "<style>" element affects the whole email, including my response as I compose it. As a result, when I press Enter, I no longer see the extra line spacing, and Enter or Shift+Enter results look the same in the HTML version of my email. Yet they insert different content, and in fact the plain text version of the email still adds two line jumps for paragraphs vs a single jump for line breaks, and I cannot tell while composing. Then, when I send my email, the recipient email reader (e.g. GMail or Office) appears to somehow pick up the difference again, so my paragraphs are displayed again with extra line spacing. This is an issue, because I cannot tell the difference between paragraphs and line breaks, they look the same to me, and I end up using both interchangeably. But they don't look the same to the recipient, and the resulting response looks messed up. How can I prevent the quoted email from overriding styles in my own response? I tried installing the CustomCSS addon to add my own "<style>" element, but unfortunately it gets added at the top of the document, and the quoted response's "<style>" element comes afterwards, thus overriding anything I set. Perhaps the sender's configuration is at fault (they should not use a global "<style>" like this), but they are a customer, and it would not be appropriate for me to ask them to fix their email configuration. The only "solution" I can think of is to switch to "Body text" as default style, such that paragraph blocks are never used. I think it is a shame to loose this feature, but at least there is no surprises.