Fixing Thunderbird’s Column Widths Across Multiple Screens (with CSS)

If you use Thunderbird on both a laptop and an external monitor, you might have noticed something frustrating: when you move Thunderbird’s window between screens, your column layout doesn’t stay consistent.

For me, the message list column kept becoming way too narrow on my laptop screen. I wanted Thunderbird to keep my favorite layout – roughly 10 % folders / 30 % message list / 60 % message preview – whether I’m working on my laptop (1920×1080) or my larger external monitor (2560×1440).

After some digging into Thunderbird 140’s internals, I found that the new “Supernova” UI uses CSS Grid with two internal variables:

• --folderPaneSplitter-width
• --messagePaneSplitter-width

By setting these in a userChrome.css file, you can define your own default layout.

Step 1: Enable legacy CSS customization in Thunderbird

Thunderbird hides this ability by default. To enable it:

1. Open Settings → General → scroll to the bottom → Config Editor.
2. Search for: toolkit.legacyUserProfileCustomizations.stylesheets
3. Set it to true.
4. Restart Thunderbird.

This tells Thunderbird to load custom CSS from your profile.

Step 2: Find your Thunderbird profile folder

The quickest way:

1. Go to ☰ → Help → More Troubleshooting Information.
2. Under Profile Folder, click Open Folder.
3. You’ll end up in a path like: C:\Users\<YourName>\AppData\Roaming\Thunderbird\Profiles\<profile-id>.default-release\

Inside this folder, if there’s no chrome subfolder yet – create one manually.

Step 3: Create the stylesheet

In the chrome folder, create a new text file and name it:

userChrome.css

(Make sure Windows doesn’t add “.txt” to the end!)

Step 4: Add this CSS

Paste the following code into userChrome.css:

@media (max-width: 2000px) {
  body#paneLayout {
    --folderPaneSplitter-width: 15% !important; 
    --messagePaneSplitter-width: 30% !important; 
  }
}

@media (min-width: 2001px) {
  body#paneLayout {
    --folderPaneSplitter-width: 11% !important; 
    --messagePaneSplitter-width: 49% !important; 
  }
}

Step 5: Restart Thunderbird

Completely close Thunderbird (including any tray icon) and reopen it. When you move Thunderbird between screens, it automatically adapts the proportions.

This tiny tweak completely changes the Thunderbird experience if you work across multiple screens. It’s stable, reversible, and works perfectly in Thunderbird 115 → 140 and possibly newer.