Thunderbird does have a slightly uneasy relationship with HTML markup. First, the Tools->Options->Display settings are just that; for DISPLAY. They don't affect what goes into your composed messages. These display settings are most relevant for plain text messages. Plain text doesn't have any instructions about text formatting, so these settings are your opportunity to set the size, typeface, font family etc for such messages to suit YOUR reading preferences. If you go through to the ''advanced'' display settings page, you'll find a checkbox for "allow messages to use other fonts". This allows fancy messages to display as the sender intended you to see them. Clear the checkbox and your own choices will (by and large) prevail, inasmuch as to how they are displayed to you. It won't necessarily show the fonts being used within the message markup. The sizes are in pixels, not points. This is in line with W3C recommendations. If we all sent using "medium" then all messages would be displayed to suit the reader not the sender, which to my mind is the right way to do it. If I need larger text than you do, I want your messages shown to me at MY choice of font size, not yours. So my composition setting for "medium" allows this. Your setting an absolute type size, in pixels or points could interfere with my reading. So, on to composition. As outlined above, I'd urge you to set this to "medium" and not assert any specific text size. That leaves the choice of size to your reader. Now, I recall having set up to compose using a sans serif font and being somewhat alarmed to find that when others replied to me, my original text was being re-quoted in Times Roman. This led me to examine what goes into an outgoing message and I found that Thunderbird, despite settings that imply specific font face and size, didn't actually insert these into the message. My correspondents, using Outlook, had Times Roman as the default font and so this was being applied to my text in the absence of any specific instructions. Outlook also applied a default absolute size to my text, which looked rather silly when this default size was applied. And I'd been blissfully unaware of all this because I'd had that "allow messages to use other fonts" unchecked, and so Thunderbird was overriding the markup with my preferences. All this led me to look at what could be done in Thunderbird to set out explicit fonts and sizes. I came to the conclusion that the Stationery add-on is useful, as it allows you to set up formatting templates (using css) so applying a uniform style to the whole message. It allows you to either embed or link to signatures. It also has a very handy HTML source editor so when you get into trouble you can see the underlying source and fix up any misbehaving layout. I use the Signature Switch add-on to choose between various signatures, both plain text and HTML. Despite assurances to the contrary, I find that Thunderbird doesn't deal with HTML signatures well if sending plain text, and vice versa. And to deal with messages messed up by Outlook, I use the Extra Format Buttons add-on, which lets me set explicit sizes. Whilst this breaks my rule about NOT setting absolute sizes, it is a least worst option for repairing the damage inflicted by other email clients, and so allows me to normalize all the text in a quoted and replied message. As an aside, I'd suggest you replace the <p> tags in your signature block with <br> tags. A para tag is quite a powerful beast and allows formatting to be applied to the contained text and so carries some organizational baggage with it, whereas a line break is just that. And I suspect each <p> tag should be partnered by a </p> tag. https://addons.mozilla.org/en-US/thunderbird/addon/stationery/ https://addons.mozilla.org/en-us/thunderbird/addon/signature-switch/ https://codefisher.org/projects/extra_format_buttons/ Finally, please understand that fonts in messages are requests, not commands. Your recipient's machine will display using Georgia if they happen to have that particular font. If they don't the local default serif font will be used in its place. And some fonts that you use will be specific to your OS or even your version of the OS and may not be available on others. As a Linux user, I don't have all the standard MS fonts, so I'll never see, for instance, Georgia or Segoe.