Installation

1. Download

Download userChrome.css

2. Enable custom stylesheets

In Thunderbird go to Settings → General, scroll to the bottom and open Config Editor, then set:

toolkit.legacyUserProfileCustomizations.stylesheets = true

3. Keep the tab bar visible (recommended)

In the same Config Editor set:

mail.tabs.autoHide = false

so the tab bar (and with it the one-line layout) is always visible, even with a single tab.

4. Find your profile folder

Help → Troubleshooting Information → Profile Folder → Open Folder

5. Copy the files

Create a chrome folder inside the profile folder if it doesn’t exist, then copy userChrome.css into it.

6. Restart Thunderbird

The layout applies on restart.

The repository also contains userContent.css for the mail body, compose editor and settings page. It is work in progress and currently not recommended: it still applies the colour layer that userChrome.css no longer does, so it pins those surfaces to a grey the rest of the interface no longer uses. BirdOne works without it.