I am unclear on your issue. In settings>general, you can set a default font and size. I have variable width set, but I also have Segoe set as font and that is what I see in compose window.

Okay, this may sound strange, but here is the real way to set fonts on Thunderbird. And I attached examples of my own setting of Calibri 14 as my default. Here goes:

1. in Tools>settings>general, set your default font.
2. then click the advanced button. Notice the drop-down menu at top. It probably says 'latin' or 'other writing systems' - this is IMPORTANT:
3. select 'latin from that menu, set your desired font and font size settings, click OK,
4. select 'otherwritingsystems' from the drop-down menu and set the IDENTICAL settings to what you did for Latin, and then click OK. In attached screenshots you will see my latin and otherwritingsystem match. Notice I also set monospace font.
5. if you have account set to compose HTML, then set that in tools>settings>composition to variable-width and medium font. (You would only change the HTML font setting if your intent is for recipient to receive your preferred font - not my personal recommendation.)
6. In settings>composition, set Sending Format to 'automatic' (just my preference, not required)

NOTE: if font size is still too small, try a larger number, such as 20. the numbers in thunderbird are pixels, not points.

Thank you for your reply; I appreciate the extent of you response. I learned one or two new things from it, eg to set the 'other writing systems' identical to Latin. However it has not solved my problem. 1. (in case it is relevant) my Tools menu does not include Settings. I can access these under Edit and from the top-right three-line menu. 2. Settings/General allows me to set language and fonts as you describe, and under Advanced I have changed Proportional to sans serif, and Serif, Sans-Serif and Monospace to DejaVu Sans - these were previously different from each other, but I don't really understand what they apply to. Under Font Control I have left both Allow messages to use other fonts and Use fixed width font for plain text messages ticked . 3. The settings under Composition were already as you recommend. 4, But New Message / Ctrl+n still give Paragraph and Variable width. Closing Thunderbird and reopening it makes no difference to the New Message, and the new settings are retained.

Thank you for your reply; I appreciate the extent of you response. I learned one or two new things from it, eg to set the 'other writing systems' identical to Latin. However it has not solved my problem. 1. (in case it is relevant) my Tools menu does not include Settings. I can access these under Edit and from the top-right three-line menu. 2. Settings/General allows me to set language and fonts as you describe, and under Advanced I have changed Proportional to sans serif, and Serif, Sans-Serif and Monospace to DejaVu Sans - these were previously different from each other, but I don't really understand what they apply to. Under Font Control I have left both Allow messages to use other fonts and Use fixed width font for plain text messages ticked . 3. The settings under Composition were already as you recommend. 4, But New Message / Ctrl+n still give Paragraph and Variable width. Closing Thunderbird and reopening it makes no difference to the New Message, and the new settings are retained.

The paragraph and variable width do not apply to the chosen font. If you followed instructions carefully, what you type will be the chosen font. Thunderbird does not send font and font size to recipient unless you force that in settings>composition. So, when you compose with variable width, you are freeing the recipient to see message in his/her preference. You will see it in your font. be sure that latin and other are identical.

Yes I understood that points 5 and 6 of your original reply were not part of the solution but just helpful to know. I was just confirming that. But the fact is that the solution as given has not solved the problem, so there must be some further consideration I need to take into account.

Actually, they ARE part of the solution. I forgot to mention: you can change default to body text at settings>composition. leaving that and variable width as defaults has no impact on the font choice, so I am unclear now on what your problem is. Please provide more info or a screenshot. Seeing screenshot of the latin and other writing systems settings may also help.

Ah! What you forgot to mention was the solution: I changed settings>composition HTML style to DejaVu Sans and left Use Paragraph format unticked, and now the new message comes up perfectly. Very many thanks for all your help!

I was glad to assist. There is no measurable difference in what you see in composing. The difference is that, by not using variable-width, you are requiring recipients to read your messages in your chosen font. And that may have been your intent.

Well! You are right: my new message now comes up claiming to be set as Body Text and DejaVu, but when I actually enter text it IS still Paragraph and Variable width - and won't return to produce DejaVu until I change the font to Paragraph! It was my intention to write messages to a layout which I found the best, just assuming that it was also what the reader would see. So my original question is still open: I can successfully change the Paragraph setting and font message by message, but how can I set the default new message always to appear as I want it?

Maybe if you post screenshot of the settings>compostion setting, plus the latin and other systems panes, we can figure this out. I cannot duplicate what you are describing

David I think I have found the solution: 1. Settings/General: select default Font This seems to ensure that a new message appears with that font set. 2. Choose size (by experiment, as it is not an actual font size. 3. For both Latin and Other Writing Systems, match the entries with those entered as above, and tick the two Font Control boxes. 4. Composition

I think you've got it. :) Thanks for the feedback.