Why not.
Language Reactor's video integrations are Netflix and YouTube — that's where its subtitle engine, saved words, and Pro features (speech recognition, machine translation) operate. Disney+ has never been on its supported list, and the extension simply doesn't activate there. (Language Reactor does other things beyond video — podcasts, imported text — but for streaming, it's those two platforms.)
What works on Disney+ instead.
Two mainstream tools officially support Disney+: **Trancy — bilingual subtitles and click-to-translate on Disney+, with a permanently free tier; the budget answer and the only Firefox-compatible one. LangPanda** — full study toolkit on Disney+: clickable dictionary subtitles, word saving with audio and screenshot capture, Anki export, built-in SRS; the pick if Disney+ is part of a serious study routine rather than casual watching.
Which one, honestly:
try Trancy free first if you mainly want to understand what you're watching; go straight to LangPanda's trial if you're collecting vocabulary — its capture-to-flashcard loop is what Trancy lacks. Both cover Netflix and YouTube too, so neither locks you out of Language Reactor's home turf.
Worth knowing about Disney+ content for learners:
Disney+ carries strong dubbed catalogs — animated films with high-quality dubs and subtitles in major languages make it a genuinely good immersion source for European languages and Japanese, which is exactly why "does Language Reactor work on it" is such a common question.