Browsers

One extension covers every Firefox-based browser. What differs is where each one looks for the helper’s manifest, and a few browser settings.

caelestia-tab install writes to ~/.mozilla always, and to a fork’s own directory only if the fork’s home directory already exists. So install the browser first, then run install.

Is the helper connected?

Open Settings, Advanced on the new tab. Status shows the scheme it received, or the browser’s reason it couldn’t start the helper. The usual one is “No such native application caelestia_tab”: the manifest isn’t where the browser looks, or its path points at a binary that’s gone.

The helper’s own errors (a scheme file it can’t parse, a wallpaper it can’t read) go to the extension’s console: about:debugging, caelestia-tab, Inspect.

Zen’s new tab

Out of the box Zen opens its command bar instead of a new tab page, so the new tab never shows. Set zen.urlbar.replace-newtab to false in about:config (it defaults to true in Zen 1.22.3b).

Tree Style Tab

With Tree Style Tab installed, its sidebar follows the scheme live, with the same choices as the new tab’s background (Settings, Browser):

• A colour (the default: primary, dimmed 86%): the colour dimmed towards the surface mixed towards black (40% in dark mode, 12% in light, where surfaces grey quickly), so it sits a step darker than the page in both modes and a high dim is a light tint.
• The caelestia wallpaper, dimmed towards the scheme’s background and blurred as you set, with the tabs on a translucent tint of primary. X offset and Y offset (-100% to 100%, 0 centred) move it: -100 lines up its left or top edge with the sidebar’s, 100 its right or bottom. The sidebar gets a copy scaled to 900 px, since TST keeps the style in every sidebar.
• Tree Style Tab’s own look.

The fields are the background’s: the colour shows when the source is a colour, and the blur and offsets when it’s the wallpaper. A tint set up before this, with its Strength, carries over as the same colour dimmed by 100 minus it.

It goes through TST’s own API: caelestia-tab registers with TST and hands it a stylesheet, so it needs no permission, and nothing happens without TST. The wallpaper option is unit-tested for the CSS it produces (tests/extension.test.mjs); how it looks in a real sidebar is unconfirmed.

Zen’s own window

Zen ignores the WebExtension theme API: a test extension that set every theme colour to something garish recoloured Firefox completely and changed nothing in Zen 1.22.3b (2026-09-27). So CaelestiaFox doesn’t reach Zen’s window either. caelestia-tab colours it with a Zen mod, kept live by a small autoconfig script; see Zen’s window.

Tested with

Checked on 2026-09-27, each with the extension loaded through web-ext run, the helper registered by caelestia-tab install, and live wallpaper switches through caelestia wallpaper:

YouTube’s cookie-consent overlay, shown to a logged-out visitor, keeps its own colours; the catppuccin style doesn’t cover it.

Chrome and Chromium

Not yet. The extension uses the browser.* namespace, and Chrome’s service worker background has no DOM, which the bundled less.js expects. Chrome support is planned in IDEAS.md.