Where and how much of the message font changes?

If you were to place the cursor back into previously typed text, does it show the original font selections, or the replacement?

I'm wondering if the action of adding an attachment moves the cursor and the insertion point, so that the font indicator falls back to the default.

Needless to say, I've never seen or noticed this happening. But then I don't tend to change font on the fly. Or I type first and format afterwards.

I add attachments before adding text. As soon as I place the cursor in the text section the font box changes to variable width. I then have to go to font box to change it back to default font.

How do you set your "default font"?

Mine is done via a Stationery template. So, before I start typing, it shows me the default font as determined in Thunderbird's settings (which I don't care about, because Stationery will set what I actually want to use.)

As soon as I put the cursor into the writing area i.e. inside the zone controlled by the css formatting declared by Stationery, I see my preferred font settings.

I am surprised in your case that if you have set default font in the Tools|Options|Composition window that there would be anything other than your defaults at work. IOW, where does it get the "wrong" front settings from?

Have you ever inspected your html-formatted messages to see what's being encoded onto them?

Font is set as you say. Not sure how to see what's in encoded message. If I add attachment after doing text, all is fine, but if I add before doing text then it changes from default to variable width, which is not any form of default font.