Free SEO Extension for Firefox

Download for Firefox

The DiagnoSEO SEO Extension for Firefox is a free add-on that brings the same on-page audit, GEO / AI Overviews readiness checks, and HTML vs rendered DOM diff to Firefox users — with full feature parity to the Chrome version. It is one of very few SEO extensions that supports Firefox natively rather than treating it as an afterthought.

Why Firefox users specifically benefit

Most popular SEO extensions are Chrome-first or Chrome-only. SEO Pro Extension is Chrome-only entirely. AITDK SEO Extension is Chrome-only as of 2026. Detailed SEO Extension, SEO Minion, and SEOquake have Firefox versions, but feature parity is often delayed. DiagnoSEO was built with Firefox as a first-class target from the v3.3 release:

• Identical feature set. The Chrome ZIP and Firefox XPI ship from a single codebase. Every tab, every check, every keyboard shortcut works the same.
• Manifest V3 with Firefox-specific tweaks. The Firefox manifest declares browser_specific_settings.gecko with a proper add-on ID and strict_min_version "109.0". Background scripts use the scripts array fallback that Firefox prefers, paired with the service_worker entry that Chrome requires.
• Respects Firefox privacy defaults. The Firefox manifest declares data_collection_permissions: ["none"] — Mozilla's new disclosure standard that confirms zero data leaves the browser. No tracking, no analytics, no remote configuration.
• Works alongside Firefox's built-in tools. The Inspect Accessibility panel, View Page Source, and the Firefox-exclusive 3D View remain available next to DiagnoSEO's tabs — the extension does not interfere.

One-click audit with a quantified score

The Overview tab is the first thing you see when you click the DiagnoSEO icon in Firefox. It opens in under 500ms and presents the full on-page state: HTTP status, X-Robots-Tag header, title and meta with live character counts, canonical resolution against the document's base URI, Open Graph and Twitter Card preview, favicon detection. A 0-100 score collapses every signal into one number using transparent deduction logic.

Firefox's strict default content blocking does not interfere with the audit. The extension reads the page through standard WebExtensions APIs that Firefox supports identically to Chrome.

Interactive heading tree

The Headings tab renders an interactive H1-H6 tree of the current Firefox page. Each heading has a level badge (H1 to H6, color-coded), dashed indentation connectors, and a collapsible chevron. Double-click any heading to highlight it on the live page — Firefox's scrollIntoView with the :focus-visible pseudo-class handles this exactly like Chrome.

The tree flags structural issues that flat lists miss: missing H1, multiple H1s, skipped levels (H2 directly to H4), and empty headings caused by JavaScript that swaps text after render. The headings sub-section counts how many appear in each level.

Internal, external, and redirect chain analysis

The Links tab presents a 4×3 grid of link statistics: total links, internal vs external split, nofollow / sponsored / ugc / dofollow breakdown, missing rel="noopener" on target="_blank" links, empty anchor text count, and duplicate URLs vs duplicate anchors. Each card is clickable for a filtered drill-down.

The redirect chain visualizer captures every hop the current Firefox request went through via the webRequest API — which Firefox implements with identical semantics to Chrome. Each hop is color-coded: 301 and 308 green, 302 / 303 / 307 yellow, 4xx and 5xx red. Chains of 4+ hops escalate to a warning. The full link list and duplicate analysis exports as CSV.

GEO / AI Overviews readiness — unique to DiagnoSEO

The GEO tab is what makes DiagnoSEO different from every other SEO extension. It checks the on-page signals that determine whether AI search engines — ChatGPT, Perplexity, Google AI Overviews, Copilot — will pick the page up as an authoritative source.

• llms.txt detection — fetches /llms.txt and /llms-full.txt and reports existence + structure.
• Paragraph length analysis — flags paragraphs over 100 words that reduce LLM citation likelihood.
• Sentence length analysis — flags sentences over 20 words.
• AI-friendly schema validation — FAQPage, Article, Organization, speakable.
• First-paragraph number check — pages with a number in the first paragraph receive substantially more LLM citations.

Installation on Firefox

Until the add-on is approved on addons.mozilla.org (AMO), the .xpi file linked above is unsigned. Unsigned extensions install only on specific Firefox builds:

• Firefox Developer Edition — install directly from the .xpi.
• Firefox Nightly — install directly from the .xpi.
• Firefox ESR (unbranded) — install directly from the .xpi.
• Regular Firefox release — currently does not allow unsigned add-ons. Wait for AMO approval (typically 1-7 days) and install from the AMO listing. This page will update with the direct AMO link once approval lands.

To install in Developer Edition or Nightly:

1. Download the .xpi file.
2. Open about:config, search for xpinstall.signatures.required, set it to false.
3. Open about:addons and drag the .xpi onto the page (or use the gear icon → "Install Add-on From File").
4. Accept the permission prompt. DiagnoSEO appears in the toolbar.

Privacy on Firefox

The DiagnoSEO Firefox add-on is 100% free, no account, no tracking. It declares data_collection_permissions: ["none"] in its Firefox manifest — the new Mozilla disclosure standard that confirms zero data leaves the browser. Network activity is limited to: HTTP header reading from the page you are actively viewing, one favicon URL fetch for preview, two optional /llms.txt checks when the GEO tab is opened, and one raw-HTML fetch of the current URL when the Diff subtab is clicked. All requests go exclusively to the site you are currently visiting. Nothing reaches a DiagnoSEO server.