Mobile Inspector by Luiz silva

Firefox for Android has no developer tools. No console, no element inspector, no network tab — and no add-on can build a real DevTools, because the devtools.* API does not exist on Android. If you need to see what a page is doing on a phone, you have two options today: a USB cable with a computer next to you, or nothing.

This add-on is the third option. The inspector runs inside the page itself, on the device itself.

INSPECTOR

Engine: Eruda (MIT licence), bundled inside the add-on. Nothing is fetched from a CDN.

• Console: logs, errors, warnings and a command prompt.
• Elements: walk the DOM with the selected node highlighted on screen.
• Network: XHR and fetch calls, with method, status, timing and body.
• Resources and storage: cookies, localStorage, sessionStorage, scripts, stylesheets and images.
• Sources: the text of the files the page loaded.

You enable the inspector per site — until you enable it, it stays idle. Once a site is enabled, the inspector loads with the page at document_start and survives refreshes and navigation; that is why the Network tab catches calls from the very start of the load, not only the ones that happen after you open the panel. If you would rather not enable the whole site, you can open it on the current page only, once.

PAGE INFO

Own code, not Eruda. It is a reconstruction — and an extension — of the desktop Firefox "Page Info" dialog, which does not exist on Android.

General: title, URL, host, origin, scheme, content type, size, encoding, doctype, language, text direction, referrer, referrer policy, last modified, document state, visibility, DOM node count and secure context.

Media: images, videos, audio, inline SVG, canvas and CSS backgrounds. Every item gets a real thumbnail, drawn on a canvas inside the page from the bitmap the browser has already decoded — zero network requests. Tap a thumbnail for the floating preview (dimensions, type, alt, address), and there is a button to download the original file.

Links: total, internal, external, anchors, mailto, tel, javascript:, nofollow, noopener and "_blank without noopener", grouped by host. That last one is a security finding that routinely slips through review.

Scripts and stylesheets: what the page loaded, and from where.

Security: negotiated TLS protocol, cipher, key exchange, signature scheme, certificate transparency, HSTS, EV, ECH and the full certificate chain, with PEM export on demand. This block reads webRequest.getSecurityInfo() and shows more detail than desktop Firefox's own Page Info. Firefox for Android does not show the site certificate anywhere; here you get the whole chain.

HOW TO OPEN IT ON THE PHONE

Three-dot menu → Extensions → Mobile Inspector. Firefox for Android has no toolbar and no context menu: that menu item and the inspector's floating button, once enabled, are the entry points.

LIMITATIONS (read before installing)

• This is a reconstruction, not the Firefox engine's DevTools. The devtools.* API does not exist on Android, so nothing here comes from the browser engine: no debugger with breakpoints, no profiler, no network throttling, no engine style editor. The Sources panel shows file text, it does not control execution.
• On sites with a restrictive CSP, the network hook may not work. The console and DOM inspection keep working. The add-on detects this and warns you on screen instead of failing silently — and it does not strip the site's CSP protection to work around it.
• The Network tab catches XHR and fetch from the start of the load, but not the resources the HTML itself requests: page images, CSS and scripts do not show up there. That is an Eruda limitation.
• A cross-origin image without CORS gets no thumbnail: the browser forbids exporting the canvas in that case. You get a tile with the file type and the reason instead.

PRIVACY AND PERMISSIONS

The add-on collects and transmits no user data. No telemetry, no analytics, no account, no server, no CDN — on AMO's data collection disclosure it is declared as "none". Everything happens on the device.

What it does with the permissions it asks for:

• injects the inspector only into the sites you enable, or into the current page when you ask;
• reads the page content (DOM, media, links, scripts, stylesheets) to build Page Info;
• asks the browser via webRequest.getSecurityInfo() for TLS, cipher and certificate chain details;
• stores locally the list of sites where you left the inspector on.

What it does not do: it never turns itself on for a site, it does not modify, inject or block page content beyond its own panel, and it does not disable or weaken the site's CSP.

WHO IT IS FOR

People who build or maintain websites and need to debug on a phone, without a computer nearby. Free.

CREDITS

Inspector: Eruda, MIT licence. Page Info: this add-on's own code.