This XPI module/bundle contains a dictionary combined of standard dictionaries:
- english (UK, EU English) and
- german (classical spelling)
Drag and drop the xpi file on a Firefox window or simply click the XPI in the releases section and confirm the installation.
Alternatively open "about:addons" as a URL, click the "gear" button and choose "Install Add-on From File...".
(Fortunately Mozilla does not require signatures on dictionaries yet.)
Right click into a text field, choose "Check Spelling", then "Languages" English (Germany).
- Icon - icon.png does not seem to be recognized. Adding it the the manifest.json didn't work.