You say "mark" as if it's a visible glyph, such as ¶ (known as the pilcrow). Is this what you mean? I desperately try to avoid pasting from any MS Office product, knowing that it brings a whole lot of ugly overhead with it. I'd use "Paste without formatting", but I recognise that often the point of such copying is precisely to preserve the formatting. I like to have an html source editor to hand in Thunderbird, so I can view html markup or imported text and tidy it up. If you're just looking for an unwanted paragraph break, you can then hunt down and delete the offending <p> or <br> tag. The Stationery add-on can add an HTML source editor to the composition window. https://addons.mozilla.org/en-US/thunderbird/addon/stationery/ There is also a smaller add-on that does a similar job, without all the overheads of that previous add-on. https://addons.mozilla.org/en-GB/thunderbird/addon/thunderhtmledit/?src=api