I'm using [Debian 9](https://en.wikipedia.org/wiki/Debian_version_history#Debian_9_(Stretch)) (Stretch/stable) with Firefox ESR 52.3.0 from the repositories. The only languages available for spellcheck are Spanish and German.

I tried to install the [English (US) language pack](https://addons.mozilla.org/en-US/firefox/addon/english-us-language-pack/), but it's not compatible with this version of Firefox. I also installed `firefox-esr-l10n-en-gb`, the English (GB) language pack from the repositories, and restarted Firefox, but English still isn't available as a language.

What do I do?