From MozillaZine Knowledge Base

You can set a specific initial font, initial font size, and default font color to be used for all messages that you compose and send.

These settings are stored in each message, so that people who receive your messages can see these text effects in the way that you see them. However, some people who receive your messages might have their own preference settings that prevent them from seeing these text effects.

How to set up Thunderbird

To do this, set up Thunderbird as described below.

General settings

• Go to "Tools -> Options -> Composition".
• Click on the "HTML Options" button and set the font, font size, and font color that you want. If you want to set a default background color to be used in messages you compose, you can also set that. Click "OK".
• Click on the "Send Options..." button and make sure that "Text Format" is set to anything except "Convert the message to plain text." Click "OK", and then click "OK" again to exit out of the "Options" dialog.

Account-specific settings

• Next, go to "Tools -> Account Settings" and select "Composition & Addressing" under the name of your account (in the left-hand pane). Make sure that "Compose messages in HTML format" is checked.
• If you have more than one account, repeat the previous step for each account that you want to use the specified font.

Composing & sending

• Compose a test message to see if the above settings have taken effect. If not, then exit Thunderbird and restart.
• When sending a message, if you are ever prompted about whether to send in plain text and/or HTML, make sure that you do not send in plain text only, since this will not preserve your font and background-color settings.
• If you ever want to compose a plain-text message, hold down the "Shift" key as you click on the "Write" button on the toolbar.

Note: A bug in the HTML Options dialog can make it display the font as "Variable Width" (the default) even when some other font is set. To set the font to the default, use the dialog's Restore button.

Further Note (Thunderbird 1.5): If the default message composition font you've set under "Tools -> Options -> Compose -> HTML Options" isn't being used when replying to a message or composing a message via "File -> Send Link" from Firefox's file menu, check the setting for your proportional font under "Tools -> Options -> Display -> Fonts -> Fonts." Keep in mind also that the font sizes set in that dialog are in pixels, not points. While there, confirm your "Display Resolution" setting as well. It it's blank, select one of the options until your choice "sticks." If necessary, select "Other" and have the system figure it out for you.

Advanced topics

The font and font size settings that you choose in this way are initial settings for each message. They only apply at the start of the message. If you change the text style while you are composing your message, then the initial font and font size settings no longer apply. If you want to use them for some more text, re-apply them manually. If you really want to specify defaults that apply to the entire message, see the next section.

The font color setting that you choose in this way is the default. It applies to the entire message, except where you change the color manually.

Setting defaults for font and font size

Thunderbird does not provide a user interface for setting the default font and font size for the entire message. You can work around this by inserting code in an individual message, or you can add the code to an HTML signature file or a template.

When you set the default font by inserting code, choose "Variable width" in Thunderbird to obtain the default font you have set (even if the default you have set is not really variable width).

Examples

Make the default font fixed width:

<style type="text/css"> body {font-family: monospace;} </style>

Make the default font size 12 point:

<style type="text/css"> body {font-size: 12pt;} </style>

Make the default font 12 point Comic Sans MS:

<style type="text/css"> body {font: 12pt Comic Sans MS;} </style>