Thunderbird is not a word processor, nor is HTML (the language of web pages) all that conducive the things that are designed for the printed word. Hence there are for instance no tab bar as that typographical construct has not real "place" in an online document, even less so for decimal tabs, tables being the obvious replacement. Likewise the word concept of a paragraph mark does not translate well to HTML either. Word like windows used a carriage return line feed pair to signify a paragraph which word displays as &para. Linux uses only a carriage return and HTML uses tags like < p > Basically there is no control code to display a paragraph in Thunderbird as it is implemented as pressing the enter key and is represented usually as a space line between paragraphs. You can force or suppress this line by changing the style from the toolbar in the compose window between Body Text and Paragraph. I permanent switch to one for the other can be had by changing the setting to Use paragraph format instead of body text in options > Composition