Since this is the first result on Google, I wanted to add a “better” solution I found.
Since Firefox 65, there is an option in the settings to change the UI language of Firefox. However, this option is hidden on Firefox Nightly (and apparently also Firefox Developer Edition) because language packs are not officially supported for Nightly versions. They might lead to UI crashes, however I’ve not encountered any. Proceed at your own risk.
Set these settings to true in about:config:
intl.multilingual.enabled
intl.multilingual.downloadEnabled.
You can now change the UI language in Settings -> General -> Language.