I don't generally write emails in html so I don't notice this, but it's a question that's been asked many times on the Thunderbird support forums.

A few years back Thunderbird made a change that seems to have confused and upset a lot of people. It changed the default style in the compose window (the style that the first word you type is in, if you don't change it) to 'Paragraph'. So in the email it's sent with a 'p' tag. This seems entirely logical to me - you're starting a paragraph, unless you specify it's a heading or something.

Apparently before that change, the compose window assigned 'body text' - I assume that's what it said in the pull-down list at top-left. But there is no html tag or style called 'body text'. What it did was not put any tags round the text. It just put (line break) tags at the end of each line. People had got used to that.

More confusing, for my anyway, is that they left 'Body text' as an apparent style in the pull down list. But if you select it it generates 'p' tags anyway. And it reverts to Paragraph when you hit newline. This seems inconsistent to me; I don't know whether anybody has tried justifying it.

How this appears to the recipient depends on what his app/client/webmail displays for paragraphs, and for text without tags. You won't know for sure, but it's fairly standard. Paragraphs are usually displayed with a gap between them, which is good for actual paragraphs but not so good for single lines. (So don't put an extra linespace between paragraphs: it's not a typewriter!) Untagged text is just displayed in some default font and spacing - but usually single-spaced.

It's possible to revert to the old behaviour in options/preferences > config-editor > set mail.compose.default_to_paragraph to false

What I do, if I have to compose in html, is use the Preformat style and then set the font-family, size, etc to whatever I want.

The other thing to know is that, unless the email includes italics, colour, a specific font, or something that can't be presented in a plain text email it won't sent it in html anyway - it'll send in plain-text. So that can affect how the viewer sees it. An email with two black lines of text may look different, in terms of spacing and typeface, to two blue lines.