Space Control (Thunderbird/Betterbird Add-on)

A Betterbird/Thunderbird MailExtension that lets you choose which spaces you keep. Address Book, Calendar, Tasks and Chat can each be switched off; Mail and Settings always stay.

Switching a space off removes it from the spaces toolbar and the pinned-spaces menu, hides its menu entries and toolbar buttons, disables its keyboard shortcuts, and closes any of its tabs that are open (including ones that come back through session restore).

Verified on Betterbird 140.13.0esr-bb25 (Thunderbird 140 ESR) and Thunderbird 153.0.1 — see Compatibility.

What it looks like

The spaces toolbar with everything switched on, and with Calendar, Tasks and Chat switched off — Address Book kept:

Before	After

Picking what to keep, from Tools → Space Control…:

How it works

WebExtension APIs cannot touch the main window UI, so the add-on ships a small Experiment API (api/implementation.js) that runs with chrome privileges:

• CSS. One user stylesheet, registered through nsIStyleSheetService, whose text is generated from the current settings. Selectors are qualified with XUL element names or scoped to a chrome container, because the sheet is global.
• hidden attribute. The spaces toolbar filters its arrow-key navigation on [hidden] rather than on CSS, so the six spaces buttons and their pinned-menu twins are hidden the way Thunderbird itself hides them when chat is off. Otherwise keyboard focus would land on an invisible button.
• Prefs. mail.chat.enabled is Thunderbird's real off switch for chat and is set from the panel. Switching both Calendar and Tasks off also sets calendar.itip.showImipBar to false so invitations stop appearing in the message pane.
• Tab monitor. A tabmail monitor closes tabs of switched-off spaces as they open, which is what makes session restore behave.
• Tools menu. The API adds a "Space Control…" item to the Tools menu that jumps straight to the options panel, because the Add-ons Manager route (detail view -> Preferences tab) is easy to conclude does not exist.

Turning both Calendar and Tasks off additionally applies Thunderbird's own .hide-when-calendar-deactivated class — the marker it uses for its "no calendars enabled" state — so calendar surfaces that have no obvious ID are covered too, along with the Calendar pane in Settings.

Chat is the only space that needs a restart, because it only loads or unloads at startup. The options panel says so and offers a restart button.

Disabling or uninstalling the add-on undoes everything, including the two prefs.

Install

Download the .xpi from Releases, or build it yourself:

./build.sh               # produces dist/[email protected]
./build.sh --update      # replaces an already-installed copy, then restart

For a first install, use Add-ons and Themes → gear → Install Add-on From File. Do not just drop the XPI into the profile's extensions/ directory: Thunderbird treats that as a side-load, re-applies extensions.autoDisableScopes on every start, and keeps switching the add-on back off.

Both extensions.experiments.enabled and xpinstall.signatures.required already default the right way in Betterbird 140 (true and false respectively), so no pref changes are needed.

Then open Tools → Space Control… and pick your spaces. Nothing is hidden until you switch something off.

Layout

Path	What it is
manifest.json	MV2 manifest, declares the spaceControl experiment
background.js	Reads storage.local and applies it on startup and on change
api/schema.json	API surface: apply, getStatus, restart
api/implementation.js	The chrome-privileged half
options/	The settings panel shown in the Add-ons Manager

Compatibility

Declared range: 140.0 – 153.*.

Everything this add-on hides is addressed by element ID, and a stale ID fails silently — the CSS rule simply stops matching, nothing throws. So the range is only ever as wide as what has actually been checked. Both ends were verified by extracting omni.ja and confirming that every referenced ID, item-id, class, <key> and tab mode still exists, then running the add-on and confirming it really hid things:

	Betterbird 140.13.0esr	Thunderbird 153.0.1
All 45 references present	yes	yes
Spaces buttons hidden at runtime	yes	yes
Shortcut keys disabled	yes	yes
Tools menu entry added	yes	yes
Prefs applied	yes	yes

Nothing changed between those two versions, which is a good sign for the releases in between, though they were not individually tested. To widen the range further, re-run the checks in If a Betterbird update breaks it first.

Thunderbird does not require add-ons to be signed on any channel (MOZ_REQUIRE_SIGNING is off and xpinstall.signatures.required defaults to false), so installing the XPI straight from Releases is a normal, supported thing to do — not a workaround.

The Experiment API deprecation

Thunderbird plans to stop honouring Experiment APIs on the Release channel, gated by extensions.experiments.suppressed plus an extensions.experiments.allowed allow-list. In shipped 153.0.1 those are false and "[email protected],[email protected]" respectively, so experiments still work today. The switch was announced for 153, then delayed by a year to target the 2027 ESR (bug 2042677 is still open). When it flips, this add-on stops working on Release and keeps working on ESR, Beta and Daily — and on Betterbird, which tracks ESR.

Publishing status

Thunderbird add-ons are listed on ATN, not on addons.mozilla.org. A listing is not currently an option: ATN has paused review of new submissions that use Experiment APIs, reportedly until around the 2027 ESR. Updates to already-listed experiment add-ons still get reviewed.

If that reopens, two things need attention first, both from the ATN review policy and reviewer guide:

• "No Surprises." Changing mail.chat.enabled counts as changing Thunderbird's defaults, so it must be opt-in, named in the UI that requests it, and stated in the listing description. It already is opt-in — nothing changes until a box is unticked — and the pref is restored on uninstall.
• No re-implementing built-ins. Reviewers reject custom experiments that duplicate existing APIs, and prefer the shared LegacyCSS and LegacyPrefs experiments over hand-rolled equivalents. The stylesheet and pref handling here would likely need to move onto those.

Traps that cost real time

• onStartup() is mandatory. The manifest declares events: ["startup"], and SchemaAPIManager.onStartup in ExtensionCommon.sys.mjs calls api.onStartup() without checking that it exists — and ExtensionAPI does not define it. Omit the method and the call throws inside the startup promise chain, leaving the add-on enabled but inert: listed in the Add-ons Manager with a UUID allocated, but no options panel and nothing applied. Nothing is logged where you would look. This was the 1.0.0 bug.
• A same-version XPI swap runs the old code for that whole session. Bump the version when testing, or expect a two-restart cycle.
• -headless cannot verify much: runtime.openOptionsPage() fails there, and extension console output does not reach stdout even with devtools.console.stdout.chrome. Probe with Services.prefs.setStringPref from the parent script and read prefs.js afterwards, or watch for a real side effect such as calendar.itip.showImipBar.

If an update breaks it

Everything hangs off element IDs read out of omni.ja. To re-check them after a major version bump:

# Betterbird flatpak, or org.mozilla.thunderbird / thunderbird_esr
unzip -o ~/.local/share/flatpak/app/eu.betterbird.Betterbird/current/active/files/lib/betterbird/omni.ja -d /tmp/omni
grep -n "Button\|menuitem:" /tmp/omni/chrome/messenger/content/messenger/spacesToolbar.js
grep -rn "hide-when-calendar-deactivated" /tmp/omni/chrome/

Then confirm at runtime rather than trusting the greps, because a missing ID is silent. Install into a throwaway profile with a config that switches everything off, and check that the add-on actually acted:

P=/tmp/tb-test; mkdir -p "$P/extensions" "$P/browser-extension-data/space-control@davidboulay"
printf 'user_pref("extensions.autoDisableScopes", 0);\nuser_pref("extensions.webextensions.ExtensionStorageIDB.enabled", false);\n' > "$P/user.js"
cp dist/[email protected] "$P/extensions/[email protected]"
echo '{"spaces":{"addressbook":false,"calendar":false,"tasks":false,"chat":false}}' \
  > "$P/browser-extension-data/space-control@davidboulay/storage.js"
timeout 60 flatpak run org.mozilla.thunderbird -headless -no-remote -profile "$P"
grep -E "mail.chat.enabled|showImipBar" "$P/prefs.js"   # both present = it ran

License

MIT — see LICENSE.