How do I reduce font size in mail compose in Thunderbird with CSS?

xazs · Jan 9, 2026

I'm trying to reduce the font size in Thunderbird mail compose window's message body where you write a message. I need a CSS code for that. Does anyone know?

glasskuter · Jan 9, 2026

I don't use Thunderbird anymore but I found this in my old notes...if it still works.

CHANGE FONT SIZE
tools-options-all way to bottom-config editor
in search box type 'devpixels'
right click on 'devpixelsperPX-modify
default value is -1.0
depending on screen size change this value to any value until you get size you want.
(ex: 1.0, 1.25, 1.5, etc) 1.5 is good on 27 in. screen)

xazs · Jan 9, 2026

glasskuter said:
I don't use Thunderbird anymore but I found this in my old notes...if it still works.

CHANGE FONT SIZE
tools-options-all way to bottom-config editor
in search box type 'devpixels'
right click on 'devpixelsperPX-modify
default value is -1.0
depending on screen size change this value to any value until you get size you want.
(ex: 1.0, 1.25, 1.5, etc) 1.5 is good on 27 in. screen)

Omg... my friend, I appreciate your help but I said in CSS.

glasskuter · Jan 9, 2026

Sorry about that. I missed that.

XxXxX · Jan 9, 2026

Change UI and Message Font Size in Thunderbird
In this day and age – when we are ‘ease of access’ aware – you’d think that the good folk behind Thunderbird would make it easy to change the font size of the GUI and messages in Thunderbird email client.

Sadly, they don’t; you have to mess about with ‘about:config’ and ‘userChrome.css’ and ‘userContent.css’ files to get results. Worst of all, the official advice, to use css files, fails to point out the need for an ‘about:config’ tweak for those files to have any effect.

I spent hours trying how to find out how to help an elderly user with eyesight issues consistent with their age make Thunderbird easier to use, with none of the advice working; almost by accident, I came across the need for the necessary prior ‘about:config’ tweak.

So, to help others with this problem, here’s what to do!

Firstly, in Thunderbird GUI, choose Tools > Options > General, scroll down to the bottom and click on ‘Config Editor’. Enter toolkit.legacyUserProfileCustomizations.stylesheets in the search box, and toggle its value to ‘true’. (If the boolean value doesn’t exist, create it, as ‘true’).

Then, open your user profile folder (via Help > Troubleshooting Information > Profile Folder > Open Folder) and close Thunderbird. In your profile folder, create a folder called chrome, and in it create two plain text files, called userContent.css and userChrome.css.

In userChrome.css (which changes font sizes in the UI), put the line @namespace url("Mozilla XML Namespace"); at the top, then eg * {font-size: 16pt !important;} below (and/or any other standard line of CSS as required);

in userContent.css (which changes the font size in messages) put @namespace url(http://www.w3.org/1999/xhtml); at the top, and then eg * {font-size: 18pt !important;} below (and/or any other standard line of CSS as required).

Restart Thunderbird, and your customisations should be applied.

from this link

Change UI and Message Font Size in Thunderbird – itproxy.uk blog

best of luck Steve ..

xazs · Jan 10, 2026

XxXxX said:
from this link

Change UI and Message Font Size in Thunderbird – itproxy.uk blog

best of luck Steve ..

Does NOT work. The CSS code for userChrome made the font on the mail itself big. The other code for userContent only enlarged the font size for the 'To' fields in the message compose window.

Any other suggestions?

john1955 · Jan 10, 2026

xazs said:
Any other suggestions?

Did you try an AI search? I got what looked like a good answer but I dont know anything about CSS code so I cannot tell

Google AI results

xazs · Jan 10, 2026

john1955 said:
Did you try an AI search? I got what looked like a good answer but I dont know anything about CSS code so I cannot tell

Google AI results

Yeah so i tried google AI too and the codes it provided didnt work at all. I also tried codes from gpt and same rubbish. Its crazy

das10 · Jan 10, 2026

For CSS to only affect how text in the Message in the Compose Editor is displayed at your end (and not at the Receiver's end) try somethig like this:

userContent.css (place this file in the same folder where userChrome.css is).

Code:

/* userContent.css */
@-moz-document url("about:blank?compose") {
  body {
       font-size: 8px  !important;     /* alter as required */
       }  
}

or:

Code:

/* userContent.css */
@-moz-document url("about:blank?compose") {
  body {
       font-size: smaller  !important;
       }  
}

Anyway, to target the Compose Editor, a userContent.css and not userChrome.css with the selector called @-moz-document url("about:blank?compose") appears to work for some of the properties of the Message Editor, although I don't know how well it works in practice considering that some of the properties can also be changed within the Compose Window itself when using html style.

pn: If you use a signature, you may need to delete the blank line at the top of your composition, and add it back with just an Enter key.

xazs · Jan 10, 2026

das10 said:
For CSS to only affect how text in the Message in the Compose Editor is displayed at your end (and not at the Receiver's end) try somethig like this:

userContent.css (place this file in the same folder where userChrome.css is).

Code:

/* userContent.css */ @-moz-document url("about:blank?compose") { body { font-size: 8px !important; /* alter as required */ } }

So I used this code but it doesn't work. But whats interesting is that before I started typing, the blinking cursor was large. But when I started typing, the font size stayed the same as usual. What do you suggest?

das10 · Jan 10, 2026

Also worth trying:
userContent.css

Code:

@-moz-document url("about:blank?compose") {
  * {
       background-color: red  !important;
       font-size: 4px  !important;
       }
}

If it works, remove the background-color line, and change the font-size as required.

eg: "testing" using: Settings > Composition > Font set to "Huge" , Style "html" , and the userContent.css shown above.

How do I reduce font size in mail compose in Thunderbird with CSS?

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