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There are options to increase/decrease the font size directly in the view menu. They affect everything, including the message list and they are applied instantly, no restart required.

![Screenshot of the "View" menu of Thunderbird. The sub menu "Font Size" is opened and the menu entry "Increase font size" is highlighted. The Background shows some redacted email entries](https://powerusers.codidact.com/uploads/x1fq5fc55k7mgjokf28lfqdrxo1r)

But they affect everything equally, which may not be what you want.

To only change the message list, you can do the following:

1. Open _Help_ -> _Troubleshooting Information_
2. Click on _Open Directory_ next to _Profile Directory_. This should open your file manager with the proper directory
3. Close Thunderbird
4. Create a new folder named `chrome` inside your profile directory
5. Inside the `chrome` folder, create a new file named `userChrome.css`

In this file you can override the CSS that makes up the UI of the program (generally called `Chrome`, which has nothing to do with the browser, except that's where the browser got its name from).

Example:

```css
#threadTree > treechildren {
  font-size: 14pt !important;
}

#folderTree > treechildren {
  font-size: 14pt !important;
}
```

The `!important` flag will make sure it gets priority over the existing CSS rules.

You will need to restart Thunderbird to have changes to this file applied.