Paragraph line spacing incorrect when composing emails

[MikeDG]

If I compose an email with a Paragraph (not Body text) which spans more than one line, the line spacing seems to be too small and each line super-imposes the one above.

I have searched this forum but I cannot find anything relevant, which makes me think it must be something which I'm doing wrong. However, I cannot find any way to change the line spacing for a paragraph. I have checked for updates and it seems that Thunderbird is up to date. It says release 38.4.0

Has anyone got a suggestion please,

Mike.

I've put an example in my DropBox here.