Extension Pulse, week of August 10: Firefox finally ships `sandbox`, ten years late

Chrome has had the sandbox manifest key since approximately the Bronze Age (Chrome 20, ~2012). Firefox is shipping it in 154, around August 18. Bug 1685123, confirmed on the MDN 154 notes.

Sandboxed pages get an opaque origin, no browser.* APIs, and they can use eval() without picking a fight with the default extension CSP. That's the whole point: a place to run untrusted or dynamically compiled code without giving it the keys to the extension.

If you've been maintaining a Chrome-only sandbox page and a Firefox-shaped "we eval in a blob URL and hope," you can delete the hope. Cross-browser sandbox pages are a real thing as of this train.

WECG #1054, filed July 31, is the obvious next argument: should scripting.executeScript() / userScripts.execute() be allowed into those sandboxed pages? All vendors still needs-triage. Don't build on it.

Chrome 153 is hardening two things people trip over

Both CLs merged August 7, both stable September 8:

• User scripts cannot be delivered into privileged renderer processes (CL 8201204). The userScripts API was a footgun next to chrome:// and other privileged pages. It's now a closed door. If you teach the isolated vs main world split, Chrome scripting and isolated worlds is the longer version; this CL is the "and also don't inject into privileged processes" footnote becoming code.
• Background tabs prompt for reload when extension site access changes (CL 8212642). You used to change host permissions and wonder why a tab still had the old injection set. Users now get a prompt. That's better UX and a support-ticket reducer, as long as you don't surprise-prompt a 40-tab session.

There's also EXTENSION_ICON_MINI = 20 for Android Chrome. You probably don't ship a 20px icon. 153 is when that starts to look like a missing asset instead of "the browser will scale it."

Chrome 152 final beta is August 19; early stable was called for August 12. Badge cap and browser gate from two weeks ago are on that train.

AMO source builds, still

Last week: build-for-amo. If you didn't add it, 154 shipping sandbox will not save your review times.

WEWG PR #905 (privileged / isolated / main world definitions) is merged. That's the language 153's user-script block is standing on, even if the CL predates the spec sentence.

What I'd do this week

Cross-browser sandbox page: put the key in manifest.json, load the same HTML in Chrome and Firefox 154, confirm browser is undefined and eval is not a CSP error. Then delete the Firefox-only workaround.

User-script extensions: test against 153 Beta. If you were injecting into anything that smells privileged, you are about to get silence instead of a stack.

Android icon: add 20px. It's a one-line assets change and it's cheaper than a one-star "blurry icon" review.

Still compiling a Firefox zip by hand? build-for-amo. @extenshi/cli for the security pass before you upload; five scans/month free, packs after that.

Sources

• Firefox 154 release notes for developers, MDN — sandbox manifest key
• Firefox bug 1685123
• WECG #1054, inject into sandboxed extension pages, filed 2026-07-31
• Chrome 153 milestone schedule, stable 2026-09-08

Methodology

Signals dated 2026-08-03 through 2026-08-10. Chrome 153 schedule confirmation is one calendar line, not a separate story. Catalog MAU on the Firefox-154 sandbox event is the Firefox store universe we track.

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This issue of Extension Pulse is compiled from MDN, WECG issues, Chromium Gerrit, and Extenshi catalog snapshots. We don't independently verify every upstream claim. If something here is off, tell us at [email protected].

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