Increase Mozilla Thunder Bird Font Size

Posted By: dstp | Published: May-21-2020 | Updated Oct-01-2020

Thunderbird is a capable and stable e-mail client and thankfully the Mozilla foundation has decided to keep working on. One complaint you hear however is that the text is too small. Follow this how-to to increase the Thunderbirds interface font size. Especially usefull on today's large monitor sizes.

1. Create a folder, name it "chrome"" and place it in your Thunderbird profile folder which is located at
"C:\users\your user here\AppData\Roaming\Thunderbird\Profiles"

2. Note that my profile is named z8ocp07h.default, yours will be unique and equally non memorable

3. Inside that folder create a text file named userChrome.css

4. Copy the following code into that file

Add the following code into that file: You can copy directly from the text provided below

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); *{ font-size: 11pt !important;}

Restart Thunderbird for the changes to take effect. Done!

Update [Oct-01-2020]

It appears that as of Thunderbird 68+ the above technique no longer works. However after some searching a post on the Mozilla Forum contained the following simple steps.

1. Click on Tools in on the Menu

2. Select General on the left hand menu

3. Scroll all the way to the bottom until you see "Config Editor"

4. With editor open search for pixels

5. Search will return "layout.css.devPixelsPerPx"

6. Double Click the line and an editor pops up

7. Change the line from -1 to 1 or 1.2

The changes takes place immediately, so make changes until your happy with the results.

Starting with ThunderBird version 78+ you have to enable this by setting the preference toolkit.legacyUserProfileCustomizations.stylesheets to TRUE. Use the following Steps

1.Tools > Preferences

2.Scroll to the bottom and click on Config Editor...

3.Search toolkit.legacyUserProfileCustomizations.stylesheets

4.Set the above option to true.

Now you can experiment with both options until you get your desired results