birdone
one line for everything
tabs and unified toolbar in a single row and a findbar on top
layout only · no extensions · only CSS
the theme – one line for everything
BirdOne merges the Thunderbird tab bar and unified toolbar into a single row: tabs on the left, toolbar buttons on the right, next to the window controls. The chrome shrinks to one line, and your mail gets the vertical space back.
Below 850px window width the layout falls back to the default two-row interface, so a narrow window stays usable.
The whole theme is pure CSS. One userChrome.css stylesheet plus an optional userContent.css – there is no extension, no theme, no .js or background process. Toggles switch individual features on and off without touching the rest of the stylesheet.
BirdOne is being reduced to what it does best: layout, not a colour scheme. The former amber accent and the surface greys are switched off – what remains is a quiet grey for the message list. A companion userContent.css is in the repository but currently not recommended.