Type size in Thunderbird

First of all, read this: http://kb.mozillazine.org/Bad_Eyesight_-_Thunderbird

...and this: http://kb.mozillazine.org/Font_settings_in_Thunderbird

Please see my notes below on setting fonts for multiple character sets. Note also that the MailTweak extension mentioned in the first link is, sadly, no longer maintained.


If you have difficulty reading the list of incoming messages and the folder pane where your accounts are listed, then this add-on:

Theme Font & Size Changer

can help you by setting the font used in these displays.

Message Text

When you start writing a new message, Thunderbird will open a composition window using the default composition mode set for the active account. The default mode is set here:

Tools|Account Settings|<select account>|Composition & Addressing

Holding down the shift key whilst you click on write (or reply or forward) will open the message in the alternative mode.

The plain text option

The simple way to address font sizing issues is to compose in plain text. You set your display to suit you. The message contains no hidden formatting or layout instructions (other than white space such as carriage returns and tabs) and your correspondents are free to display it to suit their needs. You cannot specify fonts, typefaces, or colours in the received messages, so it's very much mean-and-lean and minimalist.

You have some modest formatting capabilities:

what you type

what it looks like







Of course, you can mix and combine these decorations and use BLOCK CAPITALS.

Plain text results in smaller messages that use less space, upload and download faster, and cannot carry any unwanted payload (scripts, webbugs) in the text and so are fundamentally safer than formatted text. You can attach images and so forth, but you can not place them inline inside the text.

Formatted text

Thunderbird offers an HTML composition mode. This provides a toolbar with various formatting options.

Further stylings are available on the menu, such as strikethrough, subscript and superscript.

Thunderbird doesn't offer the option to let you set absolute font sizes. The reason for this is that email is an electronic communications medium, where your message will be seen, by and large, on a display screen. You as sender don't know anything about the device your correspondent will use. He may read your message on a smartphone's tiny screen, and later elect to show it on a large display screen during a meeting. In either case, your instruction to show it in Times Roman at 10pt is meaningless; the message text will be scaled up or down as appropriate to suit the display device (can a smartphone actually display Times Roman?) and the needs of your correspondent. Someone with failing eyesight may want it in large text and possibly in a specific choice of colours and typeface.

You can specify larger and smaller typefaces to help with emphasis. You can of course use the decorations offered by the toolbars and menus, but do bear in mind that your correspondent can elect to ignore all these settings. It is not safe to rely on text decorations to make your point.

The way that formatting is added to an email message is by use of HTML tags. Here's a sample of the type of HTML code that might be used to request Helvetica font:

<span style="font-family: Helvetica,Arial,sans-serif;">Please see my notes below on setting fonts for multiple character sets.</span>

On this Linux computer where I'm writing this web page article, "Helvetica" is the default sans-serif font. If you don't have that font, then your browser will use Arial. If for some reason you don't have this very commonplace font either, then your browser is invited to use whatever is your system's default sans-serif font. My point is that an email message doesn't contain any formal description of the type style to be used, just a request to use whatever is available. Using an obscure font selection that you have had to download is pointless, because it is most unlikely that your correspondent will also have it. So their computer will substitute what is considered to be the best match. Fonts are given broad classifications such as serif, sans-serif, cursive, fantasy or monotype, but some of the "best matches" are somewhat arbitrary.

The default font used when you compose HTML emails is set here:


The fonts used to display messages on your screen are set here:


and note that these two settings don't necessarily have to agree! You may well compose in Times Roman and yet view your messages in Arial.

The size setting is in pixels, not points. Pixels are generally smaller than points, so you need more of them; try half as much again.

10 pt ≈ 15 px

12 pt ≈ 18 px

14 pt ≈ 21 px

If you don't want to have to accept other people's choice of fonts, clear the box "Allow messages to use other fonts".

You need to repeat these settings for each character set you use. For English speakers, you need to look at Western, Other Languages, Unicode and maybe User Defined, plus any that you may use with correspondents who use particular encodings. I've highlighted this with the red outline.

The Other Languages option has become significant in that it may be used for messages encoded in Unicode, which allows many different human languages and their alphabets and scripts to be encoded using a single character set.

An email message should contain a declaration of which character set was used in its composition, and a recipient's email client should honour this request. Occasionally a message is malformed or is internally inconsistent and may declare one encoding system yet actually use another. If you have strange characters in a received message, you may be able to correct this by overriding the declaration in the message here:

View|Character Encoding

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