Language Reactor for Firefox: What Works in 2026

If you search the Firefox Add-ons store for Language Reactor, you will not find an official extension. Language Reactor, the popular tool that adds dual subtitles and clickable dictionaries to Netflix and YouTube, ships for Chrome and Chromium-based browsers only. What you will find on the Mozilla store is an unofficial third-party port, uploaded by an independent developer, that the Language Reactor community itself has questioned.

So what should you actually install if you use Firefox and want to learn a language from Netflix or YouTube? This guide covers the real situation with Language Reactor on Firefox, the risks of unofficial ports, and the alternatives that support Firefox natively.

Is There an Official Language Reactor Firefox Extension?

No. As of August 2026, Language Reactor's official extension is distributed through the Chrome Web Store, and the developers have not published a Firefox version.

There is a listing on the Firefox Add-ons store called "Language Reactor sacrosanctx". It is not published by the Language Reactor team. It is a port uploaded by a third party, with a single released version, and users on the official Language Learning with Netflix forum have asked whether it is legitimate precisely because nothing about it is endorsed by the original developers.

Why Unofficial Ports Are a Gamble

• No update path: Netflix and YouTube change their players constantly. Official extensions ship fixes within days; a one-version port can silently stop working and never recover
• Unclear code provenance: a subtitle extension reads everything on the page it runs on, so an unofficial repackage means trusting an anonymous uploader with that access
• No support: if captions stop loading or your saved words disappear, nobody is accountable

None of this means the port is malicious. It means you cannot verify what you are running, and for a tool you use daily on logged-in streaming accounts, that trade-off is hard to justify when native Firefox alternatives exist.

What Firefox Users Can Install Instead

The good news: the core Language Reactor experience, dual subtitles, instant word lookups, and vocabulary saving, is available on Firefox through extensions that support it officially.

FluentAI: Official Firefox Support

FluentAI publishes an official Firefox add-on alongside its Chrome version, so the Firefox build gets the same updates as the Chrome one whenever streaming platforms change. It covers the same ground as Language Reactor and goes further on platform support.

• Dual subtitles on Netflix, YouTube, Prime Video, and Disney+
• AI word analysis: click any word for meaning, grammar, and pronunciation in context
• Text-to-speech for individual subtitle lines
• Spaced-repetition review of the words you save
• Interactive readers for articles and books, so practice is not limited to video

asbplayer: Open Source, Anki-Centric

asbplayer is an open-source subtitle player popular with the Anki community. It works on Firefox and can sideload your own subtitle files onto streaming video. It is powerful but manual: you supply subtitles yourself for many platforms, and the workflow is built around mining sentences into Anki rather than watching with integrated lookups. A good fit if you already run an Anki-centric routine and do not mind configuration.

Switching Browsers for Study Sessions

Some learners keep Chrome installed only for Language Reactor sessions. It works, but it splits your history, passwords, and saved vocabulary across two browsers, and most people stop bothering within a few weeks. If Firefox is your daily browser, an extension that actually supports Firefox beats a second browser you resent opening.

Language Reactor vs Firefox-Native Alternatives

• Official Firefox support: Language Reactor ✗, unofficial port ✗ (third party), FluentAI ✓, asbplayer ✓
• Netflix and YouTube dual subtitles: Language Reactor ✓, FluentAI ✓, asbplayer via your own subtitle files
• Prime Video and Disney+: Language Reactor ✗, FluentAI ✓, asbplayer via your own subtitle files
• Clickable word lookups: Language Reactor dictionary popup, FluentAI AI-powered analysis, asbplayer basic
• Built-in review: Language Reactor basic saves, FluentAI spaced repetition, asbplayer exports to Anki
• Updates when platforms change: Language Reactor fast (Chrome only), unofficial port one version ever, FluentAI fast on both browsers

How to Set Up Dual Subtitles on Firefox

1. Install a Firefox-native extension such as FluentAI from the Mozilla Add-ons store
2. Open Netflix or YouTube and start any video with subtitles in your target language
3. Enable dual subtitles: your target language on top, your native language below
4. Click words you do not know to see meaning and pronunciation, and save them
5. Review saved words later instead of pausing your show to take notes

Frequently Asked Questions

Is Language Reactor available for Firefox?

No. Language Reactor officially supports Chrome and Chromium-based browsers only. The "Language Reactor sacrosanctx" listing on the Firefox Add-ons store is an unofficial third-party port with a single released version, not a product of the Language Reactor team.

Is the unofficial Language Reactor Firefox port safe?

There is no public evidence it is malicious, but it is unverified, unmaintained, and unaffiliated with Language Reactor. Subtitle extensions run on pages where you are logged in, so most users are better served by an extension whose developer officially supports Firefox and ships updates.

What is the best Language Reactor alternative for Firefox?

FluentAI is the closest like-for-like replacement with official Firefox support: dual subtitles on Netflix, YouTube, Prime Video, and Disney+, AI word lookups, and built-in spaced-repetition review. asbplayer is a strong open-source option if you prefer an Anki-based workflow and manual subtitle files.

Does Language Reactor work on Prime Video or Disney+?

Language Reactor is built around Netflix and YouTube. For dual subtitles on Prime Video and Disney+ you need a different extension; FluentAI supports both, on Chrome and Firefox.

Will Language Reactor release a Firefox version?

The developers have not announced one. Firefox's WebExtensions API supports this category of extension, as the native alternatives show, so the limitation is a product decision rather than a technical one.

The Bottom Line

Language Reactor does not run on Firefox, and the unofficial port on the Mozilla store is a one-version third-party upload you cannot verify or count on. Rather than switching browsers or gambling on an unmaintained repackage, install a tool that treats Firefox as a first-class platform. FluentAI's official Firefox add-on delivers the dual-subtitle workflow Language Reactor users expect, extends it to Prime Video and Disney+, and adds spaced-repetition review on top.