The official answer.
Language Reactor ships for Chrome and Chrome-based browsers. There is no official Firefox version, and users have been requesting one on the Language Reactor forum for years without a release.
The unofficial port — know what you're installing.
A third-party repackaging of Language Reactor exists on Mozilla's add-on store (listed under a modified name by an independent developer). Being unofficial means: it isn't maintained or endorsed by Language Reactor, it can lag behind the real extension when Netflix or YouTube change their players — which happens regularly in this category — and you're trusting a third party's build of an extension that reads your streaming pages. We don't recommend relying on it for anything you care about, and we don't link it for that reason.
The maintained Firefox options.
Trancy officially supports Firefox (and Brave, Chrome, Edge), covers more platforms than Language Reactor — YouTube, Netflix, HBO, Disney+, TED, Coursera, Udemy — and its free tier is the most generous in the category, which makes it the natural Language Reactor substitute for Firefox users. The trade-off to know: no Anki export on any Trancy tier. If Anki export is essential to you, the honest answer is that the serious study tools (Language Reactor Pro, LangPanda, Migaku) are all Chrome-family — meaning a second browser for study sessions may be the pragmatic move: keep Firefox for everything else and run study sessions in Brave (the least-Google Chrome-family option) with the tool of your choice.