BirdOne

One-line layout for Thunderbird: tabs and unified toolbar share a single row, global search hidden (use CTRL+K).

Tested on Thunderbird 152 (Supernova UI, 115+ required) on Windows.

Features

• Tab bar and unified toolbar merged into one row: tabs on the left, toolbar buttons on the right, next to the window controls
• Responsive: below 850px window width the layout falls back to the default two-row interface
• Fixed amber accent (#fabd2f) decoupled from the Windows accent colour – selection highlights, primary button and focus rings stayamber regardless of the OS setting
• Colour as signal (dark mode): context menus, app menu, toolbar icons, column headers, recipient pills and compose-window controls answer hover with amber text/icons instead of a background block.
• Message list selection, the "New Message" button and the header/compose toolbar buttons use a subtle amber tint with a 1px amber outline instead of a solid amber block
• Square popup corners: no rounded inner corners in context menus and panels
• Window controls (dark mode): minimize/maximize hover in amber glyph colour instead of a grey block; close keeps the Windows close-red, as glyph colour instead of a red block
• One surface grey (dark mode): folder pane, message list, reading pane and the dark-reader mail body share a single grey (#282828, --birdone-surface) instead of the mixed near-black defaults

Installation

1. Download userChrome.css and userContent.css

2. In Thunderbird go to Settings → General, scroll to the bottom and open Config Editor. Search for toolkit.legacyUserProfileCustomizations.stylesheets and set it to true.

3. Recommended: in the same Config Editor set mail.tabs.autoHide to 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. Create a chrome folder inside the profile folder if it doesn't exist, then copy userChrome.css and userContent.css into it.

6. Restart Thunderbird. The layout applies on restart.

Customisation

BirdOne is configurable through CSS variables. See all options → docs/customisation.md

Firefox?

You are looking for FoxOne!

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Concept inspired by NeroWolfe_'s one-line experiment | License: MIT