I don’t know if this is the place to put this, but I am having trouble with the built-in editor.
When I am composing a complex message, I find it easier to use Open Office Writer for composing… Because of my eyesight, I use a larger font size. When I copy my text from Open Office to the email edit page, it not only pastes as very small type (my default in my Thunderbird settings is 16pt) and the space between paragraphs is about 4 lines deep. If I try to enlarge the pasted type, nothing happens with the line height and the word wrap overlaps the lines. I have not found any way to correct this in the Thunderbird edit screen.
It would be very nice if there were a true wysiwyg editor addon.