Please follow these instructions precisely:
1) Visit the Mozilla Dictionary Page, then click "Download Dictionary" for your desired language.
2) Scroll to the bottom of the page, then click "Version Information" at the bottom of the page, then click "See complete version history >>"
3) Find the latest version that is NOT "webext," then click the "+ Download Now" button.
4) In Postbox, navigate to Preferences | Options > Composition > Typing > Edit Dictionaries.
5) Browse to the downloaded dictionary file on disk.
Your default dictionary preference can be set in Preferences (macOS) or Options (Windows) > Composition > Typing > Language.
Within the Compose window, you can quickly switch between dictionaries for the current compose session by clicking and holding the Spelling toolbar button. Once you send, the dictionary will reset to its default language.